that's a good one! I don't know how someone knitting the sz 48 could use more yardage than the person knitting the sz 60 (largest size).
I think it's a discrepancy on the part of the knitter, in keeping track of her yardage! Or, something amiss with the Rav software...like maybe she clicked on the wrong yarn, OR the Rav software 'computed' the total yardage for the sweater with different yardage data. For example, sometimes a particular yarn brand currently has 109 yds per ball. But older balls of the same yarn used to have 210 yds. Rav software might not discriminate between the two. So when she enters 12 balls, Rav software gives a total yardage of 1308...when in actuality, her older yarn gave her 2,520 yds!
There is nooo way sizes 44-60 could be knit 'at correct gauge' with 1300 yds.
But, as to your question: how many yards will you need to knit the sz 56? The pattern says 3,294. (18 hanks of Tahki Donegal Wool x 183 yds each) You want to knit yours 3" longer? Be on the safe side. Purchase 20 skeins. That will give you 3,940 yds. I think the skeins have 197 each, times 20=3,940 yds.
It is much better to have leftover yarn! If you purchase it from a local store, you could take back the extra skeins!
Another thought: how comfortable are you with buying just 10-15 skeins?
Are you sure you can still get your color and dyelot if you run out?
A gauge discrepancy can also throw off people's "yardage claims" in their Rav folder, too. I wouldn't put too much stock into yardage claims. There are too many variables.
I highly suggest that you "overbuy" your yarn...and take back the extra. Or, find another use for it! Such as scarves, hats and mittens for yourself, or for gifts.
Let us know what you decide to do! Happy Knitting!