Every vector knows how many elements it contains. Call numbers.size() to get this count. Returns zero for empty vectors, updates automatically when you add or remove. Vectors also have capacity, memory allocated.
Call numbers.capacity() to see this. Capacity is often larger than size because vectors allocate extra to avoid frequent reallocations. You rarely worry about capacity. The vector handles it.
But understanding the difference shows why vectors are fast: they don't reallocate every time you add one element.