The answer is: As many as you like!
Tama generations can go on FOREVER, FOREVER, FOREVER and the tama will NOT shut down or anything once it has reached a "certain maximum". However, yes, the generation counter will only show 99G at the max. You cannot find a generation counter that reads 100G or even 999, 999G. I recall that dancinkayley (another member) once had a tama that reached its 104th generation. He had been running it for almost three years. The generation counter will NEVER go above 99 but you CAN have as many as you like. Usually the tamas from 100th G and up will be odd-generation characters, though some have reported getting even-generation characters.