A Nidhi Company is a company which is essentially based on the concept of engaging in non-banking financial lending and borrowing services between its members only. Its main source of funding comes from the contribution of its members. It does not need RBI approval as compared to NBFCs for setting up, therefore making it a relatively easier option to go for. Nidhi companies are regulated by the Ministry Of Corporate Affairs (MCA) and have to comply with Nidhi Rules, 2014 and Public Limited Company norms, since it is always incorporated as a public limited company.
As for the second part of the question – Simply put, you need 7 members and 3 directors to start a Nidhi Company.
However, it is compulsory for a Nidhi company to have 200 members within a year of its existence.