I think that game stored the saves in the folder:
C:\Users\(Username)\AppData\LocalLow\https___www_patreon_com_Milfarion\Milf's Resort
although when I played it apparently it had more slots available
And the easiest thing to do when you run out of free slots in games that have few to store the saves, is to create a shortcut to the folder where the game store the saves, to open and select that folder or all the saves stored in it, then open the context menu with the right mouse button, select a compressor that you have installed like WinRAR, and save that batch of saves or the folder that contains them inside a compressed file, and number that archive by date or in some way that allows chronological sorting of the backed up saves in this way. And overwrite old saves already backed up with new saves when you run out of available slots, whether or not you delete the old saves after backing them up into a zip file.
And when you want to load a batch of old saves having backed up the current ones as well, select the zip or rar containing them, open the context menu and select extract here to overwrite the current saves with the old ones, and do the same to reset the current saves.
And the Unity version of Milf's Resort is an abandoned game, so you have to resort to some trickery if the game doesn't have a mod to fix the problem.