It really depends what you want to do.
Make a vector of length 5, filled with ones:
std::vector<int> vec(5, 1);
Grow a vector by 5 and fill it with ones:
std::vector<int> vec;
// ...
vec.insert(vec.end(), 5, 1);
Or resize it (if you know the initial size):
std::vector<int> vec(0);
vec.resize(5, 1);
You can also fill with elements using one of the many versions of fill, for example:
fill_n(back_inserter(vec), 5, 1);
and so on…. Read the library documentation, some of these functions return useful information, too.