Ehhh...
how many gallons??? That's kind of a big variable.
Assuming it is five... five gallons of 1070?
ok... what kind of beer??? What kind of yeast?? (two more big variables)
If you're doing a big wheat (I doubt it... not that big) you might want to underpitch. The strain on the yeast tends to give some folks the "offlfavors" they actually WANT in a wheat.
If you're doing a straight pale or an IPA and are using a clean US-05 or something, yeah, maybe one packet of dry would work but it is probably a much better idea to do two. Why underpitch (which will lead to all sorts of off flavors you don't want) when you don't have to?