They don't need one number, they need two (or more) that are related in such a way they have cryptographic properties. And the math says to derive them from a s

They don't need one number, they need two (or more) that are related in such a way they have cryptographic properties. And the math says to derive them from a seed. However the seed must be secret because it's a back-door. Luckily everybody can generate their own seed and their contributions can be combined without knowing the seeds. All zeros won't work because they don't have the properties.