The number of ornaments or decorations that you wish to hang on your Christmas tree depends on the size of the tree, its height, size of ornaments, your taste, etc.
If it is your first tree or your wish to create inspiring decor, you can choose medium to small-sized ornaments. Going for extremely large ornaments will look awkward and impact your Christmas decor.
Since Germans decorated evergreen trees using apples and called them paradise apples due to their representation with the forbidden fruit of the Bible. You must use a Christmas ornament that is less than the size of an apple.