In a set of sentences, how could I determine the fewest sentences that contains all characters?
This is an instance of set cover.  It is NP-hard, so there is no efficient algorithm to find the absolute optimal solution.  The problem has been studied and there are many techniques out there, which you can find by searching on set cover.
One approach is to use a heuristic to select a subset of strings that might be a little larger than absolutely necessary, but which can be found efficiently.  See, e.g., the greedy heuristic and others derived from it.
Another approach is to use a SAT solver or ILP solver to find the optimal solution.  This is not guaranteed to terminate within your lifetime; it might, or it might not.  Some solvers will let you run for a fixed amount of time and output the best solution found so far.