Earlier quoted context omitted.
20 lands is a little low and is only appropriate if you have a very aggressive deck, with very few cards costing 3 or more mana. Conventional wisdom suggests going for around 40% lands. So 24ish lands for 60 card decks, 17ish for a 40 card deck.
My "blue bertha" decks were 40% to 45% lands. They were usually 100+ cards and consisted almost exclusively of 6+ casting cost creatures. These were only used for fun, mostly for other players to test their new decks against, never in any competition. Having more than 35% lands leads to the very mana flooding complained about in the root comment.
https://www.channelfireball.com/articles/how-many-lands-do-y...
With 35% lands (21/60) you are likely to end up mana-screwed (not enough lands) unless you build your deck to match the low land count. For a generic rule of thumb I think it is better to recommend a bit more lands than that.