Providing that the crossover is an active one (i.e. a cased piece of equipment that you plug into the mains with XLR/jack inputs and outputs) you can run as many amplifiers as you want from it – hundreds if you like. All it does is split the signal into different frequency bands – what you do with the signal after that is up to you.

To know how many speakers you can run off each amp you will need to look at the figures. What is the input impedance of each speaker (it’s usually printed on the back of the speaker)? What is the minimum load that the amp will happily drive (again, this is usually on a sticker on the back of the amp)? If your speakers are 8 ohms and the amp will go down to 4 ohms then you will be able to drive two speakers from each amp channel (since connecting two 8 ohm loads in parallel gives a combined impedance of 4 ohms). Linking any more than two speakers to a single channel in this setup would risk damaging the amp. Also bear in mind the rated output power of the amp – there is no point connecting two speakers per channel if the amp is not powerful enough to drive them properly.

In bigger systems, professional amps often have a ‘link through’ connector that allows you to daisy-chain several units together.