The governing International Automobile Federation (FIA) is currently weighing up applications for the 13th and final slot, assuming no teams fall by the wayside before 2011, with an announcement expected this month.
Brawn, who won multiple championships with Ferrari [ Images ] as technical director before taking both crowns with Mercedes' predecessors Brawn GP last year, said any new team would struggle to get a car together in time.
"I'd suggest it's getting pretty tight now," he told reporters at the Hungarian Grand Prix [ Images ].
"We've started our car for next year. Obviously the amount of effort and resource we put in is front line.
"I think teams can get cars together in shorter periods, some of the teams have demonstrated that, but it's very difficult. So it looks pretty marginal to me to get a team together in time for next year," the Briton added.
"I think we've also got a pretty healthy grid now. Twenty four cars is a pretty good size and I don't think it's a disaster if a 13th team didn't come in. What we need to do is keep the teams we've got."
The list of would-be new teams has already been whittled down by the financial climate, with several who had expressed an interest subsequently withdrawing their applications due to lack of funding.
The latest was the US-based Cypher Group, who issued a statement on Friday.
"We remain completely committed to developing a credible and viable Formula One team and were able to raise a considerable amount of sponsorship and interest in recent months," they said.
"However, after much deliberation we have decided that the budget we have is not sufficient to allow us to pursue the project in a manner befitting the series."
Formula One's commercial supremo Bernie Ecclestone [ Images ] has also warned that there was a risk of some current teams folding.
"I would not be surprised if one or two of them did not make the end of the season. I think there are a couple of teams in Formula One who really shouldn't be there," he told the Daily Telegraph newspaper.
"All we ever want is 10 teams," he added. "Lotus is a good name. I wouldn't want to lose them. But in general this year has been a bit of a nuisance because it has cost money to keep these teams in."
Brawn was more positive about the outlook, saying that he expected the smaller teams at the back of the grid to be more competitive next year.
"I think we'll have an interesting grid next year with the teams we already have," he said.