When dealing with standard TTL, a driver gate can "drive" the inputs of approx. 10 unit loads, or 10 other gates. This number 10 means that the standard TTL driver gate has whats called a FANOUT of 10. In other words, if you attempt to put an 11th unit load on the output of a standard TTL gate, NONE OF THE OTHER UNIT LOADS WILL WORK RELIABLY. This is because the voltage level available to each unit load input goes DOWN with each load attached. If you exceed the driver gates FANOUT spec. The voltage level to each unit load input will fall below a "valid high" input for TTL.