I may want to start a little project to make little bitcoin farm, which is hopefully profitable. I want to use usb miners. And what else do I need? Rasperry pi? And how much miners i can use for each raspberry pi?
You need to do some reading. Most mining is done by Asics, which most come with a controller so you plug it in and set it up, and leave it alone.
Raspberry PI is a good controller for Avalon 4.1, compacs, lots of usb miners, etc. But you have some research to do as it's a huge range of the different kinds of miners.
Just basic check what is your electricity price? And do you have vat/import tax?
Is that cheap or expensive?
That would be very expensive on electricity. You don't stand a chance with any miner of ROI there. You could get a usb one for fun.
But buying a big miner and counting on making money there will not happen. You would need to look into hosting elsewhere/cloud