I'm not always at the limit, but when I hit it, it really hurts. I try to prune down to 5-7 spaces at a time when I'm between sprints, because even with breadcrumbs there's only so much I can keep in my head and process in an ongoing manner. But to some degree, the spaces hanging out there, tagged, are my "these are what were important to you yesterday, what are you doing about that today?"

I try to have one per project, one for general chat, and ideally a little room for throwaway searches ... that might become new projects of their own....