The number of function calls is exactly what is means: number of (sub-)function calls. There's no strict correlation between func calls and complexity. To find the most efficient implementation you should try mapping your function against different argument sets and timing it. Using the timeit module.
Calculating the complexity of a function is a too wide argument for a single answer. It depends on the algorithm itself and the complexity of the subfunctions it calls.
Here for more: https://en.wikipedia.org/wiki/Analysis_of_algorithms