Anyone whose board name suggests they might be a Tar Heels fan living among the heathen deserves a reply. Even if they show a pirate flag avatar, suggesting some unholy connection with ECU. The question is a good one, and I am interested in the replies as well, because they will probably be diverse. For example, when PADI refers to 'going pro' they are referring to pursuing the DM, Assistant Instructor, Instructor training sequence. A related attribute that might define 'professional' would be the degree of liability one incurs through their diving, and whether libaility insurance is required (it is for DMs, AIs, Instructors). 'Professional' could also imply that the diver derives most / all of their income from the endeavor. Notably, a large number (? the majority) of SCUBA instructors and DMs do not derive their primary income from diving. Certainly, a commercial diver would and should therefore be considered a profesional diver, as they dive for a living. Likewise, a public safety diver, employed by a law enforcement agency, for whom diving is the primary activity as an agency employee, would be considered a professional diver from my perspective. Would the Keanu Reeves character in The Replacements be considered a professional diver, because he was cleaning hulls for income? Hopefully, others have additions / corrections.
As for the minimum number of dives, you will get two perspectives: 1) what is required by a training agency for an individual to begin, or complete a certain level of training, and 2) what many SB folks believe should be the minimum amount of experience and individual should have. The latter issue is a subject of, shall we say, animated and enthusiastic discussion, and I won't go there. As for the agency minimums, just to give you a starting point, PADI requires an individual to have 20 logged dives to begin the DM program (the first profesional step), and 60 logged dives to be certified after completion of the program - not necessarily a lot of experience. (As I am PADI-trained, I am not agency-bashing, just making an observation that others will probably make as well.) Minimums will vary across agencies, but this should give you a rough idea of the beginning level.