Is it possible to write a function template which returns whether the number of arguments is divisible by N?

Question

Yes, it’s as simple as

template<int N, typename... Ts>
constexpr bool number_of_args_divisible_by(Ts&&...)
{
    return sizeof...(Ts) % N == 0;
}

Alternatively, you can return a more metaprogramming-friendly type:

template<int N, typename... Ts>
constexpr integral_constant<bool, sizeof...(Ts) % N == 0>
number_of_args_divisible_by(Ts&&...)
{
    return {};
}