How many zeros should be in front of two's complement model?
Two's complement is a representation of a number. Using different bases, encodings, a number (as a mathematical object) can be represented in many ways.

There is no "specified number of zeros". It is rather than, if you have 10 bits, they can represent 1024 values.
It can be unsigned numbers between 0 and 1023
It can be signed numbers between -512 and +511 if you use two's complement.

"0100000000" is indeed 256 both as unsigned and two's complement signed 10 bits numbers.