I'm not sure when using Web AppBuilder there is a limit to the swipe widget. But as an alternative you can try Media Map - it's one of the express apps that can be configured (in full setup) with a swipe tool and layer list that you can use to toggle layer visibility for swipe. I just did a quick test with 10 layers with no problems.
The app is briefly covered in Apps you can use to swipe and compare.
Thank you BernSzukalski for you answer. Regards, Tim
This is useless to me as I need two widgets on the same app, while the Media Map only allows the swipe widget
Solved!! I just found out that you can only included published layers in your swipe, and not unpublished layers.