the walls are made of bricks, and they have a width of more or less 30cm. My notebook shows 6 wifi networks in my neighborhood. I don't know if it is much or not.
6 networks is not really much (my notebook sees about 20 networks at home) but is enough to fill almost the whole frequency band with some content. And if you or one of your neighbors have DECT phones (wireless telephones), that adds to the sum too as well as mobile phones with Bluetooth activated. With walls and electromagnetic noise around the possible distance of radio devices like the nRF24 is much less than they're advertised for.