i have had more than that running nicely on Motorola iDen phones. i believe the maximum size of the total stores are dependent on the device. you could use getSizeAvailable() to get the amount of room (in bytes) available for a specific record store to grow.
I have used 7 recordstores in an application for siemens sl45i and m50 models which works fine, and as i think the limitation for it is the size available at persistant storage of mobile and not number of Recordstores. Rishi