How many servos can I run on an ATtiny85?
Below is the pinout for the ATtiny85:



You said you are already using PB1 and PB4, so you must be using Timer1 to drive these PWM pins, using the pins as: OC1A and OC1B.

It appears you also have PB0 driven by Timer0 as pin: OC0A. It would also seem you are capable of controlling 3 PWM pins independently with the ATtiny85 (PB3 is just the NOT of PB4).

Simply set these registers for Timer0 in a similar fashion as you have with Timer1.

I've included the datasheet here:
ATtiny85