Can maximal number in poset be more than one?
Consider for example the poset of all subsets of $\{1,2,3\}$ of size at most $2$ ordered according to inclusion. The maximal elements in this poset are $\{1,2\},\{1,3\},\{2,3\}$.

For an even more striking example, every antichain (set in which no two elements are comparable) is a poset in which all elements are maximal and minimal. For example, consider the poset of all subsets of $\{1,2,3\}$ of size exactly $2$ ordered according to inclusion. There are three elements $\{1,2\},\{1,3\},\{2,3\}$, and all are maximal and minimal.

It is always possible to add a maximal element to a poset so that it contains a unique maximal element. See if you can figure out how.