Quick take
A 15 GPM-class system is a strong mainstream target for many four-bathroom homes; very large fixtures or frequent simultaneous use can justify 22 GPM.
15 GPM is the common middle tier
VIQUA Arros 15 is rated 15 GPM at 30 mJ/cm² and 12 GPM at 40 mJ/cm², giving useful residential flow headroom.
22 GPM for high-demand homes
Arros 22 raises the 30 mJ/cm² rating to 22 GPM and the 40 mJ/cm² rating to 16 GPM.
Bathroom count is only a shortcut
Four bathrooms with low-flow fixtures can need less than a luxury three-bathroom home with rain heads and body sprays.
Size to the dose you care about
Do not compare UV units by a single headline GPM without checking the validated dose condition attached to that rating.