How powerful are the Tarlys? How many man can they field? Do they have bannermens? Their lands doesnt seem to be very plentiful up in the red mountains so i guess not very populated lands. Not much riches and men power to throw around.

There’s not an exact number, but they are: ancient (going back to the sons of Garth Greenhand), a principal house of Highgarden, and a Marcher Lordship tasked with guarding the Prince’s Pass. 

I would guess that they aren’t that rich (although they could probably tax trade, and I would guess have quite a few sheep and cattle), but strong in manpower because if they weren’t they wouldn’t exist.