Should be Large Size Cotton "L" Marked on Shirt..

Are there 6 shirts or you buy just one shirt? Thanks

Hello Nar, these come in packs of 6

Do they stretch out at all?

Yes they stretch out, depends on your body though...out of the drawer I usually stretch the chest / upper body out, and return to being tight out of the wash which is always nice as there's not too much fabric.

I'm 5'9, 170lbs, can a small fit me?

No way, I'm a 5'7 125lb female, I bought small and they're really tight. I would buy different tanks if I were you the material is scratchy/harsh/uncomfortable.