How many islands does Indonesia have?

There are total number of 17,508 islands of which about 6000 are inhabited. Straddling equator, the archipelago is on a crossroads between two oceans, the Pacific and the Indian Ocean, and bridges two continents, Asia and Australia.

Is Bali the only beach in Indonesia?

There are other beaches located in:
Nusa Dua Beach, Bali.
Pink Beach, Komodo Island. Pink Beach
Gili Trawangan, Lombok. Gili Trawangan.
Sire Beach, Lombok. Sire Beach on Lombok with Mt.
Trikora Beach, Bintan. Palm-lined Trikora Beach.
Nongsa Beach, Batam.
Pangandaran, Java.
Seminyak Beach, Bali.