Which cards would you recommend me? It's better to get a bigger amount of cheaper cards or getting les cards but more expensive?
I would suggest against buying at all (be sure to run your numbers first, ROI can get in the years super easily).
Most users around here mine upper midrange. As a side note, NV cards have regularly improved kernels and I'm not sure what's the current situation with AMD. In general big card is better than small, just avoid top models.
I always buy the second tier cards. I bought 7950 instead of 7970. I also buy R9 390 instead of Fury.