I had an email discussion with Russell Lyons a few years ago about maximizing the number of spanning trees among all graphs with a given number of vertices and edges. He had a simple argument for an upper bound of $(2e/v)^{v-1}$. There's an even simpler argument for an upper bound of $e\choose v-1$. Russell thought there was a good bound for regular graphs due to McKay. 

As for lower bounds, if the graph is not connected, it has zero spanning trees, and even an $n$-vertex graph with just $n-1$ edges missing (compared to the complete graph) may not be connected. I suppose one could restrict to connected graphs and then ask for a minimum.