Lot size refers to number of underlying securities in one contract.
The lot size is determined keeping in mind the minimum contract size requirement at the time of introduction of derivative contracts on a particular underlying.
For example, if shares of Company XYZ Ltd are quoted at Rs.1,000 each and the minimum contract size is Rs.2 lakh, then the lot size for that particular script is 2,00,000/1,000 = 200 shares i.e. one contract in Company XYZ Ltd. covers 200 shares.
Note that the lot size is revised by exchanges from time-to-time.