In standard writing, it is recommended to use one space after "RE" or any sentence-ending punctuation. The practice of using two spaces after a period was more common in typewriter usage but has largely been phased out in modern digital writing. Therefore, one space after "RE" is appropriate for clarity and consistency.
no spaces
0 spaces before and after a dash
2 Spaces
First you go over how many spaces then you go up how many spaces
There aren't any spaces after the apostrophe in a contraction (did you see what I did?)
how many spaces in the heart
no spaces
what do you call something that has many open spaces
135 spaces
0 spaces before and after a dash
2 Spaces
employee productivity
There are 32 black spaces on a checkerboard.
256 character/spaces
The spongy layer or mesophyll has many air spaces.
The number of spaces that follow an abbr. is one.
It used to be 2 spaces but nowadays it is only 1.