Minardi will be making an announcement sometime within the next month or so regarding their engine supplier for next year. And one of the rumored possibilities is Cosworth -- and Jordan has also been named as possibly using Cosworth power next year, too. BUT, Nikki Lauda has said that Cosworth will only supply 2 teams -- Jaguar and 1 other. Assuming Arrows is also using Cosworth next year (and, considering the way things have gone these last couple weeks, that's not completely guaranteed), there's a potential for 4 interested customers.
I seem to recall something about a proposed or upcoming rule regarding the number of engines that a team can use in a given weekend. However

I can't remember whether a) it was actually a rule or simply a proposal and b) what's the rule?
Because, if that is the case, Cosworth could find itself with excess capacity -- and supplying 4 teams could be a more viable option than it is now.