Do you really need to use 19 rounds when hunting?
Depends on the hunting. Some varmint hunters fire hundreds of rounds in a single afternoon.
Do you really think you are at risk of being attacked by groups of 20 people?
Handguns are underpowered and often require multiple good hits to stop an attacker. 20 rounds from a handgun won't be reliable defense against 20 attackers.
But yes. People defending their business from rioters may very well need to defend against 20 attackers at once.
That said, I think a reasonable compromise will be to subject the police (both federal and local) to the same gun restrictions that the populace has to comply with. If it turns out that the police are able to cope with a 10 round magazine restriction, then perhaps we can consider that it is OK for self defense too. But if 10 rounds is an unacceptable limitation for police, then it is surely too restrictive for self defense as well.