The number of voting members in the US House of Representatives is fixed at 435. That was set by Congress in The Reapportionment Act of 1929. In addition to the 435 members representing the states, there are 6 non-voting delegates: one from the District of Columbia, one Resident Commissioner for Puerto Rico and delegates from the Northern Marianas Islands, the US Virgin Islands, Guam, and American Samoa.
The seats are apportioned to the states according to the
population of each state, with every state getting at least one
representative. Currently, seven states have only one
representative. California has the most at 53. Every 10 years the
Census Bureau provides the population numbers. Some states gain
representatives and some states lose representatives but the total
number stays at 435.
The number of members of the US House of Representatives was capped at 435 in 1911. It is still based on the population by state. So if one state requires a new Representative due to an increase in size, another state must lose a Representative.