There are some per-model stats from Germany issued at the end of 1993 (so a few months before Commodore went tits up)[1], and they have ~1 million A500s, and ~100 thousand A1200s (plus circa 10k A4000s). So there's a fair few out there, but in the greater scheme of things, they're massively outnumbered by the original models (and, seemingly even the A600 outsold the A1200, at least at that point).
The A1200 was only released about a year and a half before the end of Commodore, and I doubt that there was a massive sales boost in the last few months, or the Escom phase of zombie-Amiga, so AGA is a smallish minority of Amiga in general.
[1] http://www.bambi-amiga.co.uk/amigahistory/sales.html