17 is the Mariners's elimination number, which should also be the A's magic number.
Edit: Oh, to get home field over the Yankees? Looks like 25 to obtain a tie. I haven't looked at the tie-breakers.
The A's & Yankees have split their season series. Wikipedia thinks the second tie-breaker is your division record, even if you're in different divisions.
Oakland is currently 30-34 and have 12 games left against division opponents. The Yankees are 34-26 and have 16 games left against division opponents. If the A's tie the Yankees on record (picking up 3.5 games), this could go either way.
The next tie-breaker is against AL opponents - the A's have a better inter-league record than the Yankees do, and there are no more inter-league games left for either, so if the two teams are tied, the Yankees will have a better record against the AL.
Edit: So I think 26 is the appropriate magic number for the A's to get the #1 WC slot. OTOH, if they accomplish this, there's a decent chance they pass Houston as well, who the MFY control the season-series tie-breaker with.