Before we talk about stuff like HIMARS, we have to realize what a finite resource it is. The Americans themselves only have 400 total launcher systems in service, and no one is sure how many are actually serviceable at the moment. The reload times for the HIMARS is also considerably shorter than the Russian/Ukrainian equivalents, so they fire off a lot more rockets per day even in a shoot and scoot scenario. But to get the ammo boxes into Ukraine is a logistical challenge at the moment. For every HIMARS firing unit, you would probably need multiple truckloads of ammo just for one day of consistent firing. And they have to be special vehicles too, as most likely the Americans will also send their supply trucks equipped with cranes, something the supply trucks of Russian origin do not have. Then you have to consider the type of ammo the Ukrainians are going to be using, which will probably be the GMLRS equivalent rockets. While they are the most plentiful variant, the Americans have only ever made 50,000 of them, shared between all the countries who use this system, meaning just over 8000 ammo pods worth of ammunition. If a HIMARS can fire off 10 ammo pods per day (6 rockets per pod) on a conservative basis (they can fire a lot more), that means there are only 200+ days of ammo in existence for just 4 HIMARS units, and less if you add more launcher units in the mix. Not available ammo, but we are talking about total manufactured, ever. So you can imagine the actual number of ammo available that can be sent to Ukraine will be a lot less.
It's a superb system for what it is, but in the end, ammo logistics is the biggest limiting factor. So the issue now becomes what do the West do in terms of weapon shipments? Are we pushing for a quick Ukrainian victory here, or are we preparing for a long conflict, and therefore need to slowly feed weapons into the country? The important thing in my mind is that the Ukrainians are able to hold out the Russians and prevent them from rapidly sending the country into oblivion. That's the first priority. Systems like the Panzerhaubitze 2000 are very very limited in number, and they consume 155mm ammo very quickly, more so than the M777's that are in theatre now in numbers. And artillery has a barrel lifespan limitation, as we are seeing now with the M777. They are being used so much that they are already in the barrel replacement phase. I imagine the CAESAR units will run into those issues soon too.
The Russians, on the other hand, are also running out of ammo at a much more rapid pace than before, thanks to Ukrainian efforts to destroy their ammunition warehouses recently. The farther the Ukrainians can push back their supply base locations, the harder they will find it to resupply their artillery. Think the emptying out of the Belarusian warehouses is indicative of that. But as with anything, the fog of war and misinformation means that we may not have the full picture of the Russian supply issues. However, with Russia seemingly firing missiles at civilian targets again, they may be changing their tactics in order to cover deficiencies in other areas of their military campaign, and maybe trying to push the Ukrainians to sue for peace.
Regarding the HIMARS and overall MRL systems. I already said that the numbers the Ukrainians claim they need are probably an exaggeration. They would need them to eventually push the Russians out completely -at minimal human cost for them- but it's not realistic for the west to give them that amount of systems as of now. Like the no-fly zone wasn't realistic either, but it wasn't wrong for Ukraine to ask. It is their country that's been destroyed, and their people killed, after all.
Still finite or not, the numbers of those systems that have been pledged to Ukraine are clearly insufficient. You say the U.S. have 400 of them on active service, HIMARS that is, but they have way more M270 on stock. Which, while not as modern as the HIMARS (and probably need some work to be operational), still outranges Russian artillery. The U.K. and Germany also operate these systems, and they have promised to give some of those systems to Ukraine, but also countries like France, Italy and Finland have them in stock. It's true that there are not a lot of them to go around, and that they are expensive and hard systems to learn and maintain, but what the West needs to understand, is that investing in a Russian defeat now, will be a lot less expensive than having to re-invest in the military because of Russia's continuous threat. Let alone the thousands of Ukrainian lives that they will save, which should be the primary concern.
Clearly, war is a logistical nightmare, and stocks of those types of weapons and ammo are/were in short supply, as there was no need for them in "peacetime". But we're not in peacetime anymore, whether the West wants to acknowledge it or not, and there's nothing to stop the manufacturers from ramping up production, significantly. HIMARS are high precision systems though, which means you use them for specific targets, not for barrages, and they are so precise you don't need to shoot too many rockets to hit your target, in comparison with other types of artillery. So while your concern about the lack of stock in ammo for those types of weapons is valid, it's may not as dramatic as you think. You touch on a great point about the current usage of ammo by Ukrainian forces in general though. There were big concerns about Ukraine forces running out of their soviet era artillery ammunition some weeks ago. So it's imperative for Ukraine's long-term military capabilities to switch to western systems gradually. This also implies a long-term commitment from the West not only to supply Ukraine with the weapons (and materials to maintain them) but also to a consistent flow of ammo. This is concerning for Ukraine, given how flippant western leadership is, but better than relying on their own stock and military industry.
The main problem to give Ukraine the weapons they need has not been logistics though. Although obviously not a minor concern, it has been bureaucracy and political tinkering which has consistently delayed the flow of weapons. Think of how long it took the U.S. to pledge those MLR systems because they didn't want Putin to get mad at them. Or Scholz consistently dragging his feet when it comes to delivering heavier weapons. Compare the speed in which Poland pledged and delivered the Krab 155 mm howitzers to Ukraine, and in how many numbers, or France with the Caesars, to how slow Germany delivered the Panzerhaubitze, and in so few numbers. There are hundreds of excuses you can make, but the reality is that until very recently, Germany's political leadership's commitment to fully support Ukraine's efforts has been the main obstacle. Hopefully, we have reached a turning point in that regard but I can't stop thinking about how many Ukrainian lives could have been saved if western leaders were a bit braver, and didn't try to appease Russia from the start.
