The number of Units to join a suddenly Barbarian Leader could be Level based, Wanda was left with twice her Level in Units upon the formation of her Barbarian Side.
Humm. But why? From what we know, if a side loses all its cities, the side's field units go barbarian. And it's not like barbarian units survive only as long as they have leadership. During book 1, we saw Jillan separate from her Gwiffons multiple times, most prominently when she got captured. But her gwiffons were there to meet her when Wanda set her free.
Wanda in book 0 wouldn't have had the shmuckers to pay for unit upkeep anyway, so it's not gameplay-significant, it just seems weird.
Edit: And while Wispri does make some good points in regards to consistency, I'm still not sure it makes sense overall.
Thing is, you're trying to compare the day to day existence
of Jillian's Barbarian Side (that happened to be allied to a Capital Side at the time) to the creation
of Wanda's Barbarian Side. What Jillian does and can do with Units that are members of her Side and how and why Units did or didn't stay with Wanda as her Side formed are unrelated questions.
Glossary to the print Book 1 says of Capitals only that they represent their side and a side needs a Capital to have a Treasury. A parsimonious guess then is that sides with a ruler and cities but no capital function more or less as normal, except for being pretty well booped by having to cover upkeep out of city income every turn, then probably losing any excess.
Yeah, it seems reasonable to assume a Side can hold Cities without a Capital. Excess income could probably be based in purses, but that wouldn't make up for the lack of a Treasury.