What would increase the number of senators?

If the United States were to add a 51st state, the senate would increase.

There are 2 senators for every state, so with 50 states, the U.S Senate currently holds 100 senators.

If Puerto Rico, for example, were to become a state, they would be able to elect two senators to represent them in the U.S Senate, making the U.S Senate consist of 102 senators.