I just spent some time going through this old thread, which I posted 21 months ago, and seeing all of the comments in regards to how many zoos each individual has visited. It makes for an interesting read.
I have a big European trip tentatively scheduled for the summer of 2019 and looking through the list of the 35 most popular zoos on the continent it seems as if I might be able to visit 11 of them...not bad for a Canadian! I'll definitely be touring Pairi Daiza and Planckendael in Belgium. In the Netherlands I'll be visiting Arnhem, Rotterdam, Amsterdam and Rhenen. In my latest rough draft I've added some days in Germany to my trip and the plan is to also visit Cologne, Hanover, Duisburg, Osnabruck and Munster in that nation.
I'll therefore visit 11 out of the 35 most popular zoos in Europe in terms of annual attendance on that original list, plus Antwerp, Gelsenkirchen, Emmen, Beekse Bergen, Amersfoort and quite possibly Frankfurt (a slight possibility) to make it 17 zoos that all receive at least 800,000 annual visitors. It's amazing that just 3 nations account for a large number of acclaimed European zoological parks and of course Germany itself has many more significant zoos that I'll have to see in the future...many years from now!