Silly question probably. Sorry in advance. 2ch for left and right, no? Or do I need a 4ch for front and rears?

You would need a 4-channel amp because it would then amplify all of your speakers. Each channel is dedicated to a speaker. So, if you have 4 speakers, you need for 4 channels. If you have 4 speakers and a subwoofer, you need a 5 channel or a less powerful 4 channel and a more powerful mono amp.

You could save yourself some money by buying a 2 channel amp and wiring the pairs of speakers in parallel. You would be wiring the speakers as seen on the second diagram on this page. The left front and rear speakers would be wired in that manner on the left channel, and the same goes for the right.

Be careful, however, as wiring speakers in this manner puts a different load (impedance) on the amplifier, and it will put out more power per channel. Make sure you check the specs for your speakers and the amplifier you are using before doing this.

[Edit:] Originally described series wiring, which was incorrect.

If you do use a 2 channel amp, you won't be able to use the front/back fader.