As best I recall, the number of transferable MG's was usually estimated around 175k. I seem to recall relatively little debate on that being the number +/- 10k and with the further observation that the population declines due to a number of reasons you can imagine. The big debate came in on your question as to how many folk own them. The obvious consensus was that the number of owners was very significantly less than the number of transferables, meaning most on more than 1.
I seem to recall that based on the very small sample size of those on that board, the average count was 5-10 or more which put the population of transferable MG owners likely in the 20 something thousand range out of 330m in the US.
I'm sure others will remember better or at least differently.