How many zeros in a picometer?

Answer 1

None. It's easily written as follows: "1 picometer".

If you want to express the same length in terms of meters, you'd write "0.000000000001 meter", which has eleven zeros following the decimal point.

It could also be written as "10-12 meter". If you do it that way, then you'd think it has one zero in it, but the same thing is often written as "1 E-12 meter" so you can't be positive about it.