There are different kinds of “no’s”.
The first is when you weigh the options and choose that something is not for you right now. You make the choice that you cannot do the project, take the opportunity, give the time, etc.
You say no because you choose to say no.
The second kind of no is a no out of necessity. You say no because you’ve already said too many yeses. You’d be spread too thin if you said yes, so you must say no.