There is no fixed number. Do whatever appeals to you (within reason).
0.2
The number of spaces that follow an abbr. is one.
None.
There aren't any spaces after the apostrophe in a contraction (did you see what I did?)
None!
none
2
There should be one space.
So you can see how many spaces are missing
One space follows a period after an initial.
One space is sufficient.
two should be left
Only one space follows a semicolon.