How to find total number of minimum spanning trees in a graph with n edges?
I'm hoping you misremembered the question, as the number of MSTs (minimum spanning trees) is not uniquely determined by the number of edges.  It depends on what edges are and are not present and also what their weight's are.

For example, if the graph has 21 vertices and 20 edges, then it is a tree and it has exactly one MST. On the other hand, if it has seven vertices and 20 edges, then it is a clique with one edge deleted and, depending on the edge weights, it might have just one MST or it might have literally thousands of them.

(A clique on seven vertices has 21 edges, which is only one more than your graph is allowed. If all the edge weights were the same, every subtree would be an MST and, by Cayley's formula, there are $7^5=16807$ different trees with seven vertices.)