My Balance 20X limits the number of Customized Domains to 250. I would like to have a list with about 20,000 entries. Would it be possible to increase this to thousands?
20,000 entries? This is the wrong way to do whatever you need to do.
There is no way you will want every bit of outbound traffic to be checked against a list that long.
Suggest you look into DNS filtering instead. Lots of positive noise on here about pi-hole and next dns
The list that peplink maintains for content filtering has 464,845 entries at this time. Adding a file or url link for a larger customized list does not seem like a stretch.