Um. yes. You need all the bolts/studs on the headers. Don't leave any behind.
If you completely missed one bolt/stud, just get one and put it in. No sense in changing the gasket when you didn't seal it right to begin with.
This will also jack with your tune. After I fixed my header leak my a/f went from a solid 11.4-11.5 to an awesome 12.5:1 a/f. Not sure if you have a wideband (you should) but it's something to look for.