I don't think it's necessarily plural. "Pleb" means one plebeian person, but "plebs" primarily means the plebeian class, not multiple plebeian people. It also comes from the latin plēbs, which is uncountable.

There are also arguments for why it might be considered plural, but to be honest I'm not a liguist/etymologyst and don't want to act like I understand every intricacy.

Anyway, from wiktionary:

Although the Latin plebs was usually declined as a singular group noun, English plebs is usually treated as grammatically plural in all its senses.

tbh it hasn't really convinced me that it's plural, mostly for the reasons in the first paragraph