No. Use as many as you need. Each number can be used once, and only the steps that built your closest number count towards your score.
No. You can stop as soon as you are happy with a number, and the numbers you did not touch cost you nothing.
How numbers are used up
Each step takes two numbers and replaces them with one result. That result can be used in later steps, but the two numbers you started the step with are gone. So each of the original numbers is used at most once.
Only the route to your best number counts
Your score is based on the number closest to the target and the steps that built it. A side route that did not feed that number does not add steps. Undo is different: every undo costs a step, whatever it removes. See undo and Reset.
In practice, many exact solutions leave one or two numbers unused. The generator’s own route often does too. For choosing which numbers to combine, read the Daily Target strategy guide.