There's no easy answer for that.
If your credit history is currently short (say, less than three years), I could make a case for applying for several cards at once. You don't have that much to lose in terms of average age of accounts. Yes, you will take a hit on inqs...possibly as much as 30 or 40 points. However, the damage will be very temporary...in a matter of months your score will recover. After a year, inqs don't even count against you.
Going forward, you'll have 6 or 7 accounts in good standing, reporting regularly. When you do add an account (say, a mortgage or car note), your average age won't take that big a hit...you might lose half a year or so.