Understanding a famous riddle

Let $n$ be the total number of flowers. When the problem says that all but two of the flowers are of one kind, it means there are $n-2$ flowers of that kind. Therefore, $n-2$ of them are roses, $n-2$ of them are tulips and $n-2$ of them are daisies. Assuming that this exhausts the list of flowers, we can write $n-2+n-2+n-2 = n$ which gives $n=3$

All but two mean's you have got to subtract $2$ from the quantity. Let $m$ be the total number of flowers. Then you have $$m-2 + m-2 + m-2 =m$$ and then solve for $m$.