Hi Nancy and thanks for this question. 6 cats, I wish I had this many, this is my dream for the future!
So when we have so many cats, even if they get a long well, it is really common to have some issues with inappropriate urination.
Oftentimes, peeing outside of the tray will be anxiety related, especially if the cat is a rescue cat and when they share their territory with another cat. However, any cat can display this behaviour.
It sounds like you have plenty of trays, and we should ensure they are all apart from each other and in areas of the home with low foot traffic. Cats sometimes 'claim' a tray.
I'd have a variety including ones with rooves and would trial a few different litter types, as cats can be very fussy.
If you think the kitty may be stressed, consider getting a Feliway Friends adapter plug-in, often stress is the root cause of toileting issues. I'd also start her on some natural calming supplements such as Zylkene.
Ensure she has all of her own resources and plenty of spaces she can go to be alone when needed. This includes water bowl, food bowl, bed, cat tree, scratching post, toys etc. Cats dislike having to share resources and it can cause them a lot of stress.
Having said the above, it would be important to rule out underlying health causes. If this cat's poos are solid, there is no straining and she urinates a normal amount, it is likely more behavioural than medical. However, we'd want her checked over for e.g. cystitis (bladder inflammation) or bladder stones. This would mean a vet check, urine analysis and possibly a bladder scan.
All 'accidents' should be promptly cleaned with an enzymatic cleaner, to eliminate the odour and prevent re-soiling.
Hopefully this stops soon!