It's more than enough if all combinations are used. However, LTO have put some meanings into some characters which prevents usage of all combinations. For example is the first letter corresponds to certain regions ie A = Regions 1 and 2, C, R=Region 3, D=Region 4, N, P, T, U, W, X, Z = NCR, S = Government plates, etc.
Not sure if these characters are reserved for some regions: E, F, H, J, K, L, M, Q, V, Y
Also, the letters I and O are not normally given by LTO on standard plates. Apparently to avoid confusion with the numbers 1 and 0
