That depends greatly on the power of the system. Certain systems can deploy a massive 16, while most are more modest at around 4. The more antennas you add to a system, the less effective each becomes as you introduce an element of loss when you split the signal. However, the advantage of more antennas means more complex coverage and allows you to navigate around internal obstructions - one of the major causes of your poor signal.
In short, if you need lots of antennas, make sure to get a more powerful booster.