Exactly one at a time
The word space has exactly one purpose: to create horizontal clearance between two words. Likewise, the space bar has exactly one purpose: to insert a single word space.
Don’t use the space bar as a general-purpose white-space dispenser by holding it down and watching the cursor glide across the screen. Though a calming sight, it leads to anguish when formatting the document. Use exactly one word space at a time.