How water pipe sizing question

I'm putting a dual vanity where there was only a single and moving my toilet and tub/shower in my upstairs bathroom. While I'm doing all that I'm just replacing all the Polybutylene pipe with copper.

Is attached a primitive map of the current plumbing. The thicker lines are 3/4" pipe and the thinner ones are 1/2". I guess I'm surprised that the hot water line for both showers is just a 1/2" pipe. Is this correct or should I run 3/4" supply over there and then T off to two 1/2" lines?

Thanks for any help you all can offer!

- Jeremy

The shower is 2 units, the tub is 4 units.
You can have 6 units on 1/2"

Thanks. That is what I was looking for!

- Jeremy