There is no set amount of extensions that you can do. Some houses may have been extended multiple times in the past until there is nowhere else that could be extended. If your house already has an extension, it is possible to make this extension larger by extending it further or demolishing it and rebuilding it a different shape or size.
We have worked on projects where a loft had been converted in the past but had scope to be extended further under permitted development. The rules around size of extensions change over time; in some cases, the depth of extensions can be made larger than what was previously allowed. A house that has a single storey extension is difficult to extend under permitted development, but under prior approval you can extend the depth of an extension; with permitted development you are only allowed to extend up to 4m for detached houses and 3m for all other types. Under prior approval, these depths are increased to 8m and 6m and can now be added onto an existing extension.
The depths of an extension in most cases is dictated by neighbouring properties and judged on how it will impact the light and outlook of these houses. You may have extended your house a few years ago and since then a neighbour has constructed an extension which is larger than yours; because of this, more often than not you would be allowed to increase the size of your extension. This would need to be approved by the council via a planning application.