The answer to this question cannot be specified precisely. After the release of a new TYPO3 version, a subsequent release follows to update the version, to fix errors observed after the initial version release.
The main point in resolving this ambiguity concerns which extensions were used to create the site. Most often, sites are created using the News extension (authored by "Georg Ringer & Contributors"), which is among the first to be updated.
If the site was created using the supported extensions for teasers, such as fluid_style_content and the News extension, it shouldn't take too long to update the website.
To enable the RTE (Rich Text Editor), the default CKEditor extension is used. In addition to this extension, to allow images to be added to the RTE, the rte_ckeditor_image extension is installed. A potential issue here is that this extension is generally not updated quickly enough to be supported in the latest version of TYPO3; there is usually a period before it is published as compatible with the new version.
There are also other aspects, such as server configuration. When upgrading to a new TYPO3 version, it may be necessary to configure a different PHP version on the server and update the database settings.
On the link, you can find the last 3 supported versions of the TYPO3 - get.typo3.org/versions
