Can you have more than one stereo pair for surround sound? Like two pairs for at total of four speakers?

No. You can use 2 HomePod mini for stereo, or 2 of the original (which will use spacial awareness to virtualize surround + Atmos).

If you want a wireless surround system then you would be looking at options from Sonos or similar, which processes using the wired soundbar and then sends off to wireless rears (and sub if applicable).