20mm is 1/72nd scale. Sometimes gets mixed in with 1/76th scale. It's similiar to 1/32nd vs 1/35th.
My entire collection is 20mm, 1/72nd scale. All of the figures are metal which I paint and mount on pennies. The vehicles are either plastic model kits or resin kits that I built and painted, or built up Dragon 1/72nd scale vehicles. I've got over 2,000 WWII figures, US, German, British, Russian, Japanese, plus close to 500 vehicles. I've got boatloads of buildings, bunkers, trenches, pillboxes, etc that are either model railroading kits that I built and "kit bashed" or they are resin models used for wargaming that I painted up. Quite a few of the items I have are one of a kind or are out of production and no longer available. For example, the Brandenburg Gate in the photos is no longer available and I've been offered as much as 300.00 for it. I'd never sell it as I would never be able to replace it. I've also got about 2,000 ACW figures and about 2,000 Napoleonics.
20mm is an extremely popular wargaming scale, so there are tons of items available in that scale. I was drawn to 1/72nd scale because as you can see, you can do a lot in a smaller space. I love to set up huge displays and leave them up for weeks at a time and just look at them from all different angles.
Thanks to all of you for the positive feedback; Louis, I'd love to sell you a display, but the problem is most of my scenic pieces are one of a kind and I couldn't sell any of them as I would never be able to replace them. I am currently working on a series of factory buildings that are model railroading kits for a massive Stalingrad display I plan on doing in the future..........