Not sure we need any cap since only owners are specified in the to: line. I guess last 5 active owners adds a sanity limit, but do people generally have more than 5 owners. Also, perhaps all owners should get the PM.
The larger issue IMO in the @mention in the body of the owner notification email. A workaround is here.
This @mention issue and who gets notified isn’t really related to this group feature, but the specific PM generated could use a fix.