I’ve collected signatures for ballot access and petitions and the rule of thumb that anyone involved in such a drive adheres to is collect twice the number of signatures that you need. Double is almost always enough of a margin.
Double would be half the number of people who voted in the past governors election. Quite a hurdle.
Another question. If the spoilage rate of a sampling of signatures is so high that an invalid petition drive can be extrapolated, can the recall petition be trashed, or will it just warrant inspecting every single signature for validity?