Vincent19:
If I use external crystal because I want to use it at 16MHz, then it means I lost 2 IO pins ?
Yes. Two pins are required for an external crystal.
The ATtiny85 is capable of running at 16 MHz using the internal oscillator. You do not need an external crystal for that speed. (The internal oscillator is less accurate and less stable than a crystal. If you need an accurate and/or stable clock then you will have to use a crystal.)