Can maximal number in poset be more than one?

In poset maximal number is defined as:
An element 'a' belongs to 'A 'is called a maximal number if there is no element 'c' in 'A' such that a is less than c.

but it again says that there can be more than one maximal number. How can it be possible?

Yes, it is possible for a poset to have more than one maximal element.

For example, let $R$ be the divides relation on the set $A = \{1,2,3,5\}$. Then $2$ is a maximal element of the poset $(A, R)$ because none of the other elements is a multiple of $2$. If you draw the Hasse diagram of this poset, you'll find that there are no elements "above" $2$ in this diagram, and so $2$ is a maximal element. Similarly,$3$ and $5$ are also maximal elements. These three elements are pairwise incomparable (similar to how apples and oranges are incomparable).

The terminology partially-ordered set is justified because the elements can't be totally ordered by being placed on a straight line, whereas the set of all integers or the set of all real numbers are totally ordered under the $\le$ relation.

StackExchange Computer Science Q#37448, answer score: 3