I did some asking about this last year. The problem is that there was a flood at the factory at some point [I think in the 1990s] and all the production records were destroyed. I believe the figures I found were that the MX was built 1981-1983 and the belief is there were around 7,000 built.
As for how many survive, there are still a lot stuffed in barns and sheds in a state of disrepair. People flew the heck out of them, go bored with them and then stuffed them away.
As for anything comparable, I think there were some other similar cruciform designs that were 2-axis. Nothing nearly as commercially successful as the MX. And with the number of MXs built, nothing that could claim the same safety record. [There were a couple similar designs that had short lifespans because they had design flaws.]
Quicksilver really got it correct very early on. Lyle Byrum was a Cessna dealer in Texas if I recall correctly when he put Quicksilver on the map. Great assembly and owners manual, unbelievable smart kit [Every part vacuum packed on cardboard sheets. It truly went together like plastic model airplane and the 80 hr build time was very accurate.] Great marketing. [He had Mario Andretti, astronaut Jim Irwin and famed aerobatic pilot Art Scholl as early owners and in very slick print ads].
As for "comparable", it depends on what criteria they are being compared on. If it is proven design, number built, number that got people aviating, ease of flying, ease of maintenance, affordability, role as an "everyman airplane", etc. Then IMHO, the MX has no peer. There has never been anything that compared to the MX as the everyman's first rung on the aviation ladder. Again, IMHO.
-Buzz