International Zoo Yearbook publishes attendnace figures.
The latest ones I have relate to 2009. I did a highly scientific analysis (if I think it is a small zoo it's on the list, if I think it's a big zoo it's not on the list), so here it is. visitor numbers vary considerably between collections.
Amazon World 50000
Birdland, Bourton on the Water 88000
Birdworld, Farnham 100000
Birmingham Nature Centre 163129
Calderglen 50000
Curragh's Wildlife Park 47311
Exmoor 48000
Galloway 28279
Hawk Conservancy, Andover 44000
Hamerton 85000
Harewood 210000
Lakeland Wildlfie Oasis 20667
Living Rainforest 80000
Mablethorpe 45000
Monkey Sanctuary, Looe 30000
Monkey World 450000
Newquay 198000
Paradise Park, Hayle 95000
Pensthorpe 55000
Raptor Foundation, Woodhurst 34000
RSCC 15000
Shepreth 83000
Skegness 117000
The Deep, Hull 384285
Thrigby 74692
Tilgate Nature Centre 250000
Tropical World, Leeds 298957
Tropiquaria 40000