Three times really for me, first was a glider with the Air Cadets, the good old Kirby Cadet mk111, one circuit was all you got anyway with a winch launch and the glide ratio. Next was a C150 at Bournemouth, in those days you had to have an actual Student pilot licence to go solo and I had to wait until mine came from the CAA! Landed to find the instructor with an arm full of mushrooms that he had been picking while I went round the circuit, don't think they would allow that at BOH these days! Last was Helicopters with HMRN, that was the only time I really noticed the empty seat beside me!
There have been other "first solos" like first tailwheel, and in many ways I did three SEP first solos as I had done the C150, then in the RN at grading you got "first solo" in the Chippie if you were good enough, then of course they considered your first Bulldog solo as a "first solo" as well!