I don't think so anymore. The old system showed percentages, but it looks like they've stopped that. If it does sell out you could probably still get a bib through a travel provider for a few days. You usually have to book 2 night at a hotel or buy a park ticket, but if you're paying for either of those anyway, the upcharge for the bib isn't too outrageous. I have a feeling that they might offer deals on some open races at the races in April and May and that will also possibly lead to a sellout.
I wish there was an option to buy insurance for $100+ bibs, but runDisney refuses. Local races are $30-45, I could register and not do 3 for less than 1 runDisney race. And now they're moving registration to 10 months before the race, where with local races the cheaper price often holds until 60-90 days before the race. Maybe if runDisney had a vastly super product, but they don't.