Unfortunately it's not quite that cut-and-dry. Maybe it used to be, but I've only been around since Feb of this year and was promoted to Master at level 40 after only 41 ranked rounds.
I wanted to stay at both the Pro and Tour Pro tiers for longer than I was allowed to, but I think I fell victim to the measures WGT put in place to combat re-starters and mulit-accounters. These measures take into account earnings, match-play results, and who knows what else. I got promoted to Master right after receiving credits for placing in a tourney, not right after finishing a round....... and I've not earned a ton of credits in tourney's.
So lesnewell, be aware that in addition to the helpful guide SilentOmegaWolf posted, you can also jump tiers (without the need for a minimum number of rounds) by placing high in a number of tournaments.
Good luck