How many aircraft and pilots can a part 135 cover?
There is no restriction on the number, type of aircraft, or pilots for a "Full 135 Operator."

There are 4 levels of 135 operators:


Single-Pilot Operator
Single Pilot-in-Command
Basic 135 Operator
Full 135 Operator *


Source: http://usac.com/faa/InstructionTwo.asp?Product=Full%20135%20Operator