Massively parallel (8 to 10 stream) music server/generator ?

[Linuxguy123]

We are considering the purchase of a house.   

It has about 8 "zones" ie kitchen, living room, bedroom, patio, library,
etc.

Each zone has 2 to 4 speakers in it, all of which are wired such that
they terminate in the utility room.

The builder of the house did not provide any wiring for an A/V
controller in any of the zones, nor an IR extender.

I'd like to build a server that generates music for each of the zones.
I'm thinking that it would serve web pages that would allow users to
select various music material and direct it to one or more zones via any
device that supports a web browser. 

On the hardware end of things, I'm thinking one could use 1 USB sound
card plus an amplifier for each zone.

How many USB sound cards could a single i5 ish PC running linux in
console mode feed ?  (MP3, wav, flac formats)

Sometimes one will want to play the same music simultaneously across
multiple zones.   Ie, during dinner, one might want to play music in the
kitchen and the dinning room.   Is there a way to sync playback between
multiple USB sound cards ?

Is there a better way to generate music for this house ?

Thanks