You can easily run multiple SDRs off of a single PI. A Raspberry Pi comes with four USB ports but they are quite close to each other so there might not be enough room to plug four SDRs directly into those USB ports. A solution to that would be to use short USB extension cables in between the SDRs and the Pi. Another solution would be to use a USB hub instead. The SDRs would then be powered by the USB hub instead of directly from the Pi. There have generally been no issues plugging three SDRs directly into a Pi. Four SDRs on the other hand can be iffy so it's probably a better idea to use a USB hub with four or more SDRs.
In case you're not aware, an SDR can be used in either Scan mode or Multichannel mode. In multichannel mode an SDR can receive multiple frequencies at the same time as long as those frequencies are all within about 2.4Mhz of each other. This means that multiple individual feeds can be set up from a single Pi if those frequencies fall within that range.
Keep in mind, that if you use multiple SDRs, each one will need an antenna connection. It's usually no problem to use a single antenna (preferably mounted outside) with a splitter so that each SDR can be provided with an antenna signal. It might also be necessary to use a signal amplifier depending on how much signal splitting takes place.
In the end, how many SDRs you use will depend entirely on how many frequencies you want to feed.
I hope this helps!
Ron
