The process-count is really not the issue here, if there is actually an issue here at all.   The first question that I would have is ... “is anything actually hurting?”   (If not, you have No Problem.)   If there actually is pain, then you need to address that by simplifying the amount of work, e.g. by storing thumbnails in a database once they have been generated the first time.   The number of processes that might be “generating a thumbnail” is probably not going to be a significant issue, although if it is, that, too, is controllable.