Why many gods and goddesses?
There are several gods and goddesses which appeared or created during different ages for specific purposes. They are paid homage, worshipped invoked at appropriate occasions.
Although there are several gods they are not the ultimate reality at all. They direct us to the ultimate reality of oneness. One destination is one and these gods and goddesses are vehicles to take us to the destination. All these deities are identical and manifestation of one.
According to Hinduism, there are 33 million gods and goddesses. In Buddhism also especially in Mahayana and Vajrayana, there are many gods and goddesses who have various duties and functions. They are also related with several mythologies.
Why some deities have many arms and heads?
Some deities are shown having many heads and arms. Some deities have several qualities.
The visual problem was solved by making images of deities with several heads and hands. Each hand is holding some objects which symbolise or represent various qualities of that particular deity. Some hands may not have any attribute but the posture of fingers and palms signify the different attitude of the god. For example, fingers when pointing towards ground means god is in charitable form. Fingers pointing upwards are in blessing pose or protection. Gestures symbolise powers.
Presence of many hands also exist. They deviate away from normal. It is mentioned that anything which deviates from normal is super normal.