how many Types of Grass and Foliage for VR and Mobile is best for performance?

How many types? Just as much as you really need and can afford performance wise.

A new grass and foliage type probably is an extra drawcall as well. Also, for foliage they tend to be very 'expensive' due to translucency / overdraw etc.
Just look at some mobile apps that have grass and foliage in it to get an idea what teams can do if they spend lots of time into this.