It is well known to use Sandwich ELISA to measure IFN-γ, which means as an antigen, IFN-γ has at least two epitopes for both capture antibody and detection antibody. Then how many does it have? or it depends on the structure of antibodies we use? What should attention to the antigens have multiple bindings in ELISA?
Well a protein can have many epitopes depending of it's structure and it's size. It's rather that antibodies which are raised again certain protein recognizes preferentially some epitopes but they could be many more.
If the epitopes are situated close to each other they might be some steric hindrance of the detecting antibodies
Any surface exposed areas of the protein that have physical room to accommodate an antibody can in principle be an epitope. The same amino acid can be part of multiple epitopes for different antibodies.
In practice, the main limitation for raising antibodies is going to be the immunogenicity of each peptide -- meaning how well is it presented on MHCII during vaccination. If you're unlucky, you're going to have one epitope dominating the immunological landscape, and you're going to have a very difficult time finding clones against any other epitopes.
So, many factors. There's not much of a point theorizing. Make your antibodies, and see what happens. Also look at the "competitive" ELISA format. It can be a bit sub-par vs the sandwich format, but it does allow you to have an ELISA with only one epitope.
The number of epitopes is in principle unlimited, as every antibody defines its own epitope (the group of chemical moieties in the antigen that interact with each antibody). Some of them are overlapping epitopes and others are totally independent . Those are the ones you need for a sandwich ELISA.
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