It depends what you mean by "at the same time". Perhaps 5-6 different side projects or so are ongoing concurrently, but not all of them are getting worked on at a given point in time. I'd say if I'm making reasonable progress in a week I'm only interacting heavily with 1-2 projects while leaving the others on the backburner. If I didn't have a fulltime job I imagine that number would change to 2-4 projects within a week (rotating projects in chunks of 2 days or so).

I certainly can end up spreading myself too thin on various projects which is why I've found cycling between projects to help. The major risk is that projects languish for months before they get the time needed to get them completed or generally up-to-date. Typically my side-project workload is about half large projects (multi-month 50+hr, typically requiring new tools or learning new skills) and half small (doable within 2 weeks under 10hr). Motivation varies month to month, so the rate of completions has a pretty high variance attached to it.

Overall I tend to yearn for more productive time to apply to projects as there's plenty of possible future projects to work on, but adding them to the queue would be irresponsible unless other projects were completed first.