There are no fixed limits.
The reasons for havin multiple stores is for orgaisational purposes or for performance purposes.
You really don't want the store to get more that 50-60GB as it can be difficult and time consuming to maintain (ie. offline defrag etc).
If you have an existing exchange server use the existing store as a guide.
As a rough rule allow a max 2GB per user, this will suggest that you have 30 users per store for a 60GB store. Exchange can handle much larger nubers than this though so you can be flexible. I would divide the number of users by 30 and this will tell you how many stores to use as a starting point.
