What a number of single-author publications means will depend entirely on what field, and as other people have noted, what sub-field you are in. In my own experience, in a field (Epidemiology) where single author papers are extremely rare, I have a modest number of them all of which fall into a single category:
These are small papers with a single idea, usually which can be addressed with a single illustrative example. Essentially they are "Dear Field, Stop Doing This. Love, Me."
Those papers don't need a second person. They are amusing side thoughts. Those don't necessarily reflect anything on the number of students I have, how friendly I am as a collaborator, or what stage of my career I'm in. The only thing they're possibly indicative of is that I'm interested enough in methods development that if that's not what you're interested in, we should possibly not work together. But the answers to what this means are too varied by field, sub-field, personal style and "Well, what's in those papers" for there to be a meaningful, general answer.