3 right now. 4 is coming out next week. I'm not sure why I never tried them before now, but I tried out the demo for 4 and was immediately hooked.

From my understanding you don't have to play all of them in order, but there are a few of the games that are connected. For example, the protagonist of 4 is from 2 (so playing 2 gives his backstory), while 3 takes place with a related character right before the events of 2. I'm playing them in an order that a friend who loves them recommended, so the order I'm playing them is this:

2, 3, 4 (these are all connected). After those three, I'll play 7 (Biohazard) and 8 (Village).

Also, now is a great time to pick them up. Other than 4 Remake that comes out next week, they are all on sale right now on the PS4/PS5.