That really depends on the TVs themselves (They traditionally provide the power for the antenna), the length of cable between all the TVs from the splitter and how far the splitter is from the antenna.
Realistically, I'd estimate that the supplied amplifier is good for 100 feet of cable. Thats a really rough, laymans type of estimation, but should work. So drop in a splitter (buy separately) and figure all the cable linking the TVs to that splitter should be at max, equal or smaller than 100 feet of cable. The clear instructions that come with this unit do show you how to add in a splitter correctly.