i think u have asked the question in a wrong way... u must have asked for the range.
why?
because atleast bounds the graph from one side but at the same time it means there is no upper bound on that .
for k=10 we have 108 spanning trees. and if u say we can have atleast 1 means that there should have been possibility for 1010 spanning trees also for complete graph with 10 vertices which is impossible.
so your question must have been as:
For a complete graph with n vertices, The number of spanning trees is at least_____?
soln=1 and that too when upper bound is not defined i mean its close to infinity for the given graph.