There is no fixed number. Do whatever appeals to you (within reason).
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
So you can see how many spaces are missing
There should be one space.
One space follows a period after an initial.
One space is sufficient.
two should be left
A diagonal cannot be a side of a rectangle, and a side cannot be a diagonal.