I've imported over 10,000 records into table with over 50 fields

might be worth checking you have all the fields

 matched to your cvs import file

also on your list above you simply may not be seeing them all
if you scroll the mouse the list above should scroll to show all fields

Well, I tried to upload a video of me scrolling through the columns, but that didn't work. Here's a screenshot...

I was able to keep adding columns after 23.

Re: indeed when I select the field 24 in the right window and drag it to the left window after field 23 it refuses to install...It bounces !

Ninox does not always like like you adding more fields to the very botton in the to the show column when you have a full list.

Try adding it near to the bottom. Then dragging it down into place afterwards
also there seems to be a bit of fine art required to get that list to scroll ! takes a bit of practice!

I didn't say it would be easy. I just pointed out it could be done.

What I do in such a situation: I add a "Line break" layout element at the bottomm of the list. I can then add whatever I need before that Line break. When the operation is completed, the Line break may be removed.

Just curious. What percent of your fields are used for each record? Are they all text fields? We have forms with over 500 fields. Depending on your use situation, you might be better off with a forms table that has a related values table. Each record in the values table would contain a single field name/field value pair. That way you could avoid a table with a lot of empty columns. Again, it depends on your situation.

The problem is that it is very difficult (but not impossible) to add an element at very end of either list when the list is filled to the bottom. And the trick is to add an irrelevant element at the end of the list as a guard. When a Line break is not possible, one can still define yet another field, and use it as the guard at the end of the list. It makes the operation of adding fields 'at the end' a bit easier, but it's surely no magic bullet.

Just another idea: you can start by moving the last field listed in the right column, and proceed from bottom to top. So, once that last field is in place, you never need to move another field to the - admitedly problematic - last position in the left column.