We recommend avoiding to delete theme copies on your own. However, if you would need to do so, we recommend deleting 4-5 of the oldest themes that have our mark as ''Backup - xx xx xx''. We also advise leaving at least 3-4 most recent themes that we marked as HC, as well as the theme that has ''Initial'' in its name.
In this way, if any issues appear, we would be able to assist you more efficiently with checking the cause of it.