They are definitely manageable, but are they happy in a flock this big? And they are going to be free range.
They will be fine.
Sometimes they may all hang out together, other times they may split up into little groups of friends.
I am sure that 20 is not too many for their own happiness.
I have all girls and one black australorp rooster. So that would be too many for him then?
If you want EVERY egg to be fertile, then that might be too many hens for one rooster.
But if you just want them to be happy, then it is NOT a problem. The rooster will not mind having that many hens available, and the hens will not mind that his attention is spread around.
Guidelines about how many hens per rooster are intended for people who want every egg to be fertile, but who do not want to feed any extra roosters. (Mostly those are people who are raising hundreds or thousands of chickens so they can sell thousands or millions of baby chicks.)