A "preferred" walkon is recruited and promised a spot on the roster, as opposed to kids that just show up via try outs or whatever.
How many roster spots are available for Walkons? Is there a NCAA regulation, or is the limit simply however many the coaches want?
Edit: So, it's like being told you can totally come and be on the practice squad for a FBS school, with a chance to earn a scholarship?
I'm not aware of any NCAA mandated limit. There is a limit of 105 players for preseason practices, but you can have as many as you want once school starts. There are also typically limits on how many players can dress for away games, subject to league rules.
The only overall limit though is how many you can pay for (uniforms, lockers, etc) and how many the coaches and staff are willing to deal with.
Of course, then there is the limited number of county scholarships, but that's a whole nother discussion.
As to their status, walkons are full fledged members of the team. They don't get the benefits of a scholarship and have to pay to eat at the training table, but they get every other privilege the scholarship kids do. That's part of why we get so many and what makes Nebraska special.