I am building a custom home and I'm wanting to know how many fixtures can share the same vent?

This depends upon the size of the vent pipe and how many fixture units you're installing on it. For instance, On a 3" vent, 30 fixture units with not over 6 water closets may be installed. A typical bathroom group consisting of a toilet, lavatory and tub/shower is 6 fixture units.
Let me know the size of the vent and the fixtures that will be using it and I'll let you know it will work by code. Regards, Tom

Thanks Tom, I am building a custom home with shower.roman tub, toilet & two lav's in the master bath. I just wanted to make sure that a 2" vent stack would be suffice.

Hi Billy,
"I am building a custom home with shower.roman tub, toilet & two lav's in the master bath."

You may connect up to 10 fixture units on a 2" stack vent.
shower=2 units
tub=2 Units
Toilet5=4 Units
Lavs= 1 unit @ for 2 Units
It all adds up to 10 fixture units. You're OK ta g!
Regards, Tom