There are 435 representatives in the House, compared to 100 senators in the Senate.
This is the number reached when new seats were added in 1913, and the number was locked by the Reapportionment Act of 1929. From 1959 to 1963, there were temporarily 437 seats.
There are 6 other non-voting delegates. There are 5 elected 2-year delegates, one each from Washington DC, Guam, American Samoa, the Marianas, and the US Virgin Islands. Puerto Rico has a non-voting Resident Commissioner as a delegate in the House, with a four-year term. The delegates have committee seats in the House and have voting rights there, but cannot vote for a bill's passage on the floor.
The reason for the limited number of representatives was to avoid a House so large that it would become unwieldy. Congressmen had every incentive to keep the number low to maintain the political power of each member. But the result has been that the size of constituencies has increased dramatically.
With one exception, the total voting membership of the House of Representatives had been increased after each of the official censuses from 1790 to 1910, at the time of the apportionments. Even though the apportionment is a Constitutional requirement, it was not done following the 1920 census. If it had been done as required, even without an increase to the total of 435 seats, 12 seats would have been transferred from one state to another, including an increase of three seats for California.
During the formation of the US Constitution the subject regarding the size of the House of Representatives came up. When a maximum congressional district size of 40,000 was proposed, George Washington, in one of his few comments regarding the debates, warned that 40,000 was too large and that 30,000 would be a more appropriate maximum. While the State of New York was debating whether or not to ratify the US Constitution, one of the delegates, Melancton Smith, warned that since increasing the number of Representatives would decrease the share of power of each existing Representative, they should not be expected to be inclined to do so. He went on to say, "It is, therefore, of the highest importance that a suitable number of representatives should be established by the Constitution."
The 12th proposed amendment of the Bill of Rights, the only one never ratified, would have established a maximum representation, and a minimum corresponding size of the House. Under that plan, the legal minimum number of Representatives in 2010 would have been 6189, including at least 12 for Wyoming and at least 746 for California. The average congressional district size is over 700,000 people, and the current population of the largest district is well over 990,000.