With the disk version of the game there is a configuration file edit to increase the number of quicksaves kept (up to 99, IIRC), but it does not work with the download version (Steam, GOG, Divinity Anthology).
Edit the config.lcl file in the '..\Divine Divinity\run' folder (in Notepad, or other text editor) and add the line MaxQuickSaveAmount # where # is the number of saves you want. If you add that as the last line, hit the Enter key before saving. The game expects the last line to be blank, and ignores it.