When the British left our country in August 1947, India was actually the British India and Indian India. The British India was consisting of 9 Governor's provinces, 5 chief commissioners provinces and some other tribal areas, Andaman Nicobar Islands etc. The Indian India consisted of some 562 Indian states including the major states such as Hyderabad, Kashmir, Mysore, Travancore, Baroda, etc. However, most of them were too small in size and too poor in resources. They all became independent and autocratic and resembled the medieval Europe. On 15 August 1947 all were technically "Sovereign".
The Indian independence act made them completely free to join either of the dominion or remain free. Except a few, which joined Pakistan, most of them decided to join India and this was unification of India in several centuries.
The accession to the Indian dominion started the new movement of territorial adjustments which included the integration and merger. Integration Involved joining of two or more states in one big administration unit such as Rajasthan and merger involved small state merging with their neighbor states such as Saraikella in Bihar.
This territorial readjustment brought the number of states under the Indian Union to 16 from the 600. The major political vision and statesmanship behind this movement was Sardar Vallabh Bhai Patel and he is also called the chief architect of Republic India.