Every two years, how many of the 435 house seats are up for reelection at the same time?

Every two years, All of the 435 house seats are up for reelection at the same time.
The House of Representatives consists of 435 members and the Federal law have the authority to determines how many members are in the House of Representatives and this law was passed on Aug. 8, 1911.A first the number of representatives are 391, due to the increase in population of United States, the number raised to 435.

That’s what i got when i looked up what the solutions were