On their forum they recon it will somewhere between 400 and 800, their fans don't travel very well even for short trips and they are whinging like fuck about the cost of the game.
They took 750 to Hereford so I would expect them to bring more to Swindon; it's marginally closer, a much better ground to watch in and a vital top of the table clash.
Their home following isn't coming out as much as might be expected as they're doing so well so any more than 1,000 is unlikely I would say.
There are a few moans about the price but I don't think it'll deter many from making a decision on whether or not to attend.