Hi:
The text field you want to set -- it is not in the repeating section, correct? Does it have to be a field (just checking -- if all you need to do is display the value, I'd use an expression box, since I already have the data in the form) - is your repeating table in your main data source or a secondary data source?
At any rate, you should be able to use a rule on the field that you are calling a "number field" -- I get a bit confused reading the description, but it sounds as if that is the field that determines which row of the table you are showing.
The rule on the number field should set the value of the text field to the desired field from the data source for the repeating table. Add a filter when you select that field -- the filter would be just like the one on your repeating table.
Post a sample of your form to a reply if you need more help with this (under the Options tab)