Part of it is the “20 different teams” wording that’s throwing some people off too. If you looking at the gamercard site, they are able to track the performance of your last 20 teams, which is probably the same for achievements. Meaning that you have to perform the achievement within the last 20 teams you used, not necessarily using 20 teams.
So, you can theorhetically do the achievement with just one team and do 50 OCVs with them. If you do 49 OCV’s and then play 20 other teams, you will not get the achievement on your next OCV because your 50 OCV’s are outside of the last 20 teams that they are able to track.