The gist is that it's a trilogy (Remake, Rebirth, Revelation), and there are some spinoffs.
It was officially confirmed as a trilogy about 4 years ago, and unofficially known as a trilogy for a couple years before that. Revelation is the final installment.
Remake and Rebirth are awesome, and you should play them.
They also remade (or "remastered", whatever) the 2007 PSP spinoff/prequel, Crisis Core, a few years ago for PS5. It's okay, but not great. Worth playing if you're really into the series, otherwise skip.
Then there's a mobile game called Ever Crisis, which sucks like only mobile games can. If you really want to catch up on obscure side stories, just read the wiki. It's way more fun.