Minimum is 1. As far as I remember (can't check just now) max is 50*50=2500 ('default * 50', like minimum is 'default / 50 = 1'), but my experience has been that like with any AI related rule change, one should not change it much from the default or the AI breaks completely.
I had found that wiki page, but the information on what exactly the values do, minimums and maximums etc. was a bit sparse so thanks for providing that additional information.
Jacew wrote:So Traits are currently rather unusable?
That, I think, is most imortant part that should be worked on in Freeciv. With Traits fixed, there should also be an option to randomize them for each civilization in the beginning. It's tiresome that every AI player acts pretty much same, and it's a lots of work to manually give suitable traits for +500 nations. Even if it would make them worse as players, it would be worth it.
EDIT: fixed some wording.
From the traits wiki page it looks like 2.6 will allow you to set a range, so setting that for the default traits would effectively be a randomise traits option.
I will try adjusting these default trait values (in 2.5) and see how it affects the AI.