they add a few each month, think 12 have been added this month plus a few ps3 games have been upgraded to the ps4 versions but some months might be more while some months might be less, it's impossible to say how many games will be added in the future and what standard they are plus in the t&c's they've protected themselves with a "*service can change anytime" type comment, when it first launched it was full of shovelware with no aaa titles but a lot of decent stuff has been added since, the ps4 games available seem to be a mix of indies & games that have been available as ps+ freebies
if you've not already played them then you'd easily get your moneys worth from the likes of uncharted 1-3, the bioshock trilogy, the borderlands trilogy, the last of us, the batman games & red dead redemption + undead nightmare, although most of those have ps4 remasters that you can pick up cheaper than a months sub so if you were only interested in the bioshock trilogy for instance it'd be cheaper just to get the remaster and play it at your own pace instead of feeling rushed to get through all 3 games in a month to get your money's worth
like i said, it's your decision, you can see what's already there and what you fancy playing, what gets added in the future is anybody's guess but there's plenty there so you might find the odd hidden gem or two that you wouldn't have played otherwise plus i think it's a rolling subscription so you can cancel anytime
