What type of tree has the most leaves and how many leaves on average will it have displayed at one time?

How long is a piece of string?
The age, size and species of a tree will be a determining factor in this. If you take a coniferous tree like a pine, each needle is technically a leaf, a very tall ancient pine would contain hundreds of thousands or millions of needles. I doubt if it is accurately quantifiable. However, a similar question was asked elsewhere and that data follows:

"It depends on the tree's species and age, but a mature, healthy tree can have 200,000 leaves. During 60 years of life, such a tree would grow and shed 3,600 pounds of leaves, returning about 70% of their nutrients to the soil."

http://abouthydrology.blogspot.com/2015/11/how-many-leaves-has-tree.html
wisconsincountyforests.com/qa-forst.htm webpage no longer exists