According to Sonos’ support site, you can use up to 6 of Sonos’ architectural speakers per Amp. But you can use up to 4 non Sonos speakers per Amp. Have a look at this page:

https://support.sonos.com/en-us/article/set-up-your-sonos-amp

I would recommend figuring out your “rooms” before settling on  the number of Amps needed for your home. Having one Amp per “room” would be my recommendation. This way you have full control over what is played in each room.

Alternately, if you didn't mind having the same audio in all rooms at the same time, a multichannel amp with a Port would be a great way to accomplish that.

You can use in-wall “impedance matching Volume controls” connected to the same amplifier (all playing the same music) to connect more pairs of speakers to an amplifier. This scheme is handy in areas, such as a large basement, where the same music will play throughout and multiple speakers will result in more even sound. This would facilitate adjustment in a local area to allow easier conversion. Locate each control near the area covered by its speakers.

Maybe consider a multi-channel, multi-source other brand amplifier and feed it with the number of Sonos Ports you need to get the number of different audio streams you want.

Probably an unpopular opinion but I wouldn’t buy Amps now as they are (I think) the oldest Sonos player currently sold new. 

IMO overdue for a hardware refresh, 100% avoid. Unless you’ve got IDGAF money then by all means.