For those watching at home. The answer is zero. There are zero MSRs in production. There have only been a limited number of test reactors, despite the research on this technology going back to at least the 1960's. Maybe someday they can be part of an answer, but in the real world, they aren't part of equation today. If people want to continue to research them, great. Have at it. But today we have renewables that produce power for a third of the cost of nuclear. And, while it's possible that nukes might get better and/or safer, it's also possible that solar will get better. In fact, while we have 70 years of history of nukes getting safer at the expense of cost, solar has always been safe and has been getting cheaper and cheaper decade after decade.