Although you can manage an unlimited number of companies in SliQ Invoicing, SliQ only loads one company at a time. This means there is no overhead in having 10, 20 or more companies showing in SliQ's recent company file list.
However, if you want to tidy up the recent file list and remove the name of a company file you are no longer interested in you will need to close SliQ then open Windows Explorer on your PC and either rename or delete the file you are no longer interested in. When you next reopen SliQ, click on the company file in the recent files list. SliQ will warn that it cannot open the file and it will remove the name from the list.