If you define a cofounder as someone who would continue the startup alone, two provide redundancy. On the other hand, if you're overly emphatic about the peril

If you define a cofounder as someone who would continue the startup alone, two provide redundancy. On the other hand, if you're overly emphatic about the peril of going it alone, then if one of two cofounders leaves, the remaining one won't want to continue.