In the example discussed in the intro, both parties entered into a futures contract to buy/sell a single apartment, hence the lot size is 1. Suppose they had entered into a futures contract that required buying/selling 2 apartments then the lot size would have been 2 and so on.

In the stock futures market, lot size refers to the number of shares of the company contained in one contract and which an investor is required to buy in a single transaction. For example, suppose the lot of size of Asian Paints is 600, then purchasing a single future contract of Asian Paints would involve 600 shares of the company. This lot size is not divisible. The stock exchange specifies the lot size and it is different for different stocks/commodities.