Over the history of the House of Commons, the number of Members of Parliament (MPs) has varied for assorted reasons, with increases in recent years due to increases in the population of the United Kingdom. There are currently 650 constituencies, each sending one MP to the House of Commons, corresponding to approximately one for every 92,000 people, or one for every 68,000 parliamentary electors.

While the Conservative – Liberal Democrat coalition governing after the 2010 general election had initially planned to reduce the number of MPs and constituencies to 600 during its term of office, Parliament voted in January 2013 to delay the boundary review this change would require. Section 6 of the Electoral Registration and Administration Act 2013 required that the next review report come by October 2018; that was done, but the delays continued. The overall measure was not implemented and later abrogated with the Parliamentary Constituencies Act 2020, from which the redrawn boundaries will be based and implemented ahead of the 2024 United Kingdom general election. The new boundary reviews were submitted on 28 June 2023.

In the 16th century there were around 310 members of parliament, including representation at various points from Calais and Wales.

In 1654 the First Protectorate Parliament included elected representation from Scotland and Ireland. After the Third Protectorate Parliament in 1659 the Scottish and Irish members disappeared from Westminster until the unions of 1707 and 1801 respectively.

In 1654 England and Wales saw its first systematic redistribution of parliamentary seats ever. However, in 1659 the representation of England and Wales reverted to the pre-Civil War pattern. When the Rump Parliament was recalled, later the same year, and the full Long Parliament was reinstated the following year its composition was exactly the same as before the Protectorate.

Only three new English constituencies, with a total of six seats, were enfranchised between the restoration of King Charles II in 1660 and the Reform Act 1832. As many of the constituencies were rotten boroughs, which had either decayed into insignificance centuries ago or had never been important settlements, whereas some major towns only participated in elections as part of the historical county they were situated in, the state of representation was very imperfect. By the Glorious Revolution of 1688 there were 513 MPs, until the Act of Union 1707 added 45 MPs for Scotland. (A total of 48 Scottish constituencies were able to return members, but six small counties were arranged in an alternating scheme whereby only three would return a member at each election, giving a total of 45 MPs). These 558 were again increased to 658 by the Act of Union 1800, with the addition of MPs for Ireland.

Over the next 85 years the number of seats varied only slightly, as constituencies were disenfranchised for corruption and the seats were re-allocated some time later (see the lists below for details). The major redistributions of constituencies following the Reform Act 1832 and Reform Act 1867 both created a new House (elected in 1832 and 1868 respectively) with exactly 658 seats. The number of MPs had dropped to 648 by the dissolution before the general election of 1885.

With the Redistribution of Seats Act 1885, introduced in the wake of the Reform Act 1884, the size of the House was increased to 670 and remained unchanged until the Representation of the People Act at the end of the First World War.