My tally is five or six, depending on how you count Nicaragua under the Sandinista regime in the 1980s – some count it, some don’t:
China
Vietnam
Laos
Czechoslovakia
East Germany
Nicaragua (?)
This time next week I’ll be in Vietnam, does that count? 
As long as your plane doesn’t crash on the way there, I’ll allow it.
Where in Vietnam? I don’t care much for HCMC, just a huge, sweltering metropolis not unlike Bangkok, but it’s good to see once. Really like Hanoi – better climate and a 1000-year-old city with a great Old Town. But central Vietnam is our favorite. A couple or three days in Hue is always pleasant, and whatever you do, don’t miss the Unesco World Heritage site of Hoi An.
Because of the impending weather patterns, we’re doing the far south, so landing in HCMC (about two and a half full days) then a bus down to Can Tho for three days then flying do Phu Quoc for a week before returning to HCMC a day and a half before we fly home.
Hoping to do central VN perhaps earlier next year when the weather is a bit more clement.
Yes, now that you mention it, it is hurricane season right now.
Also, if you tour the tunnels near HCMC, bear in mind that despite being so amazingly tiny, they’ve actually been enlarged to accommodate tourists. There is a system in central Vietnam also open to tourists that have been kept their original size if you ever want to compare.
Travelling with two little fellas (8 & 5) who have no concept of war, we won’t be doing any of the tunnels, war-museums or similar. Perhaps when they’re teenagers we might revisit the idea but at this point, nup. We’re there for the food, the countryside, people watching, dodging traffic in HCMC and then sitting on a beach in the sunshine. 
Some of these are hard to define exactly, but the ones that I think qualify are:
China
North Korea
Laos
Cuba
Syria
Transnistria
Moldova
Turkmenistan
Uzbekistan
East Germany
India (Kerala state)
I’ll be in Vietnam in January.
For me, just one (China), but I’ve been to four formerly communist countries (Croatia, Hungary, Slovakia and Slovenia).
Communist at the time? Just one, Cuba. But I was only in the American part.
I’ll be the first to say none. It’s not like an intentional boycott; I just haven’t had an opportunity.
What can I say? When you’re young, you always think all those communist countries will be there later.
How many, counting by whether they were countries at the time? Or places that were communist at the time I visited, but are independent now? Count however you choose:
1989: USSR
Russia
Ukraine
Estonia
2017:
Georgia (though I was never there when it was under Soviet control)
East Germany when I was a little kid, the same year that the whole country came crashing down. Too young to remember it (I’m 36 now).
I’d really like to visit Belarus which is probably the closest thing to a neo-communist country in Europe today. I need to learn Russian first though.
I’ve lived in Madagascar which was a “Socialist” country mildly leaning towards the eastern bloc although non aligned. I was there 15 years post Cold War though. Most people seemed nostalgic for the days of good roads and cheap rice.
USSR
East Germany
Poland
Czechoslovakia
Hungary
Romania
Bulgaria
Yugoslavia
Vietnam (although it was the South, which wasn’t commie at the time)
These were all in the 80s and work related, except for Vietnam, which was in the 60s.
The USSR and California (although the latter may be considered one of the Republics).
Russia
China
Vietnam
Cuba
Think that’s it for communist-at-the-time, other than the airport in Romania.
Kinda-sorta. I taught English for a month or so in my summer holidays in Bulgaria in 1991 which is officially the post-communist era, in that they were having free elections at that point. However, according to Wikipedia the communist or communist-lite party had actually won the previous election and was in government. This surprises me as a matter of fact, because what I chiefly remember, politically, is people grumbling about the crappy economy, and how much better things were in the Communist days
Cambridge (Mass.)
I was in Spain under Franco, but I guess that’s for another thread.
One Communist and one former communist: Cuba (the real Communist part; not the US enclave) and Czech Republic.