At the iPad 2 launch they limited it to 2 per person. I imagine its the same now, since thats also the limit the website gives.
Now that its big news that there is a massive shortage, you can be sure that scalpers are going to be among the first in line and there will be many of them. Anyone who is buying more than 1 or 2 is definitely going to be seen as suspicious by Apple and the other people in line, so be careful out there. When stuff like this happens, tempers flare and people get really upset.
In my opinion, if someone wanted one they should have pre-ordered. There was at least 3 days or so when you can preorder for launch day delivery so at this point anyone who says they desperately want one or multiples is going to have very little credibility.