You should be OK but to make sure you don't underpitch I agree with BBL_Brewer on the starter. After a while the yeast will start to die off in those smack packs and total viability of the pack is gonna go down. A well made starter will go miles in getting that pitching rate up to par.
As far as how much starter to make, once again there's Jamil to the rescue. The yeast calculator on mr malty will estimate viability based on production date, so you can ball park what kind of starter your looking at.
P.S
Depending on the gravity of the brew you might want/need to step-start it. Cheers!