Onyomi means “sound reading,” of a kanji, an imitation of the Chinese pronunciation. The opposite is kunyomi, which means “interpretative reading,” which means they found an existing Japanese word that matches (to one degree or another) the kanji.
It doesn’t make sense for any Japanese people to have onyomi last names, because that would mean they didn’t have surnames before kanji was adopted. They would just find kanji that would match their surnames in meaning. Indeed, the overwhelming majority of Japanese surnames are not onyomi. But strangely enough, some are. The ones I’ve encoutered:
“I’ve noticed myself almost all Japanese names are landmarks.”
Common people didn’t have surnames until after the Meiji Restoration (1868). Many people when choosing surnames probably chose a geographical location. For example, if you lived at the foot of a mountain, you might have chosen “Yamaguchi” 山口 as a surname.
Based on above, 五十幡 is a variation of 五十畑, which literally means 50 fields. So this is also a land name.
I really don’t have an idea. I think there are a lot of study on sirnames, but given names are arbitrary. Japanese given names are very different from English one. There are quite common names like John, Michael and etc. Japanese names are far more variant. And recently young couples name their kids with キラキラネーム =bling-bling name, distinct feature of which is its kanji and pronunciation don’t really match at all. For example: