It's supposed to be 1/8 odds to get a mythic in a pack. So with 15 mythics in a set it is 1/120 to get a specific one.
Basically this.
Too much detail:
In a big set the rare cards and mythic rares are all printed on the same sheet; this rare sheet has one of each mythic (15), and two of each regular rare (53), which adds up to (15+2*53) = 121 cards, which we know is the size of a card sheet. That makes the chance of opening
any mythic 15/121 (12.4%), which is very close to the 1/8 (12.5%) chance that Wizards quotes. The chance of opening a
specific mythic is (1/121).
In other words,if you are looking for
any mythic, you'll see one every 8 packs, but if you are looking for a
specific one (say a Tarmogoyf), you will see it every 121 packs.
(This math is for a big set, Wizards quotes the same 1/8 mythic::rare ratio for a small set but I'd wager that the numbers are slightly different).
In a typical box you would see 4.5 mythics, but Modern Masters 2 only has 24 boosters to a box, so it has about 3 mythics per box. On average, you will see one Tarmogoyf for every 5 boxes of Modern Masters opened. Since a 8-man draft uses 24 boosters (one box), it's the same math; for every five 8-man drafts of MM2 that fire one Tarmogoyf will be opened.
There's also the matter of foil mythics; for Khans of Tarkir, Wizards states that approximately one out of every 67 cards is foil; that is approximately 2 foils for every 9 boosters. It seems that Wizards doesn't print foils in the same rarity ratios as non-foils; the internet suggests that one out of every six foils is rare (or mythic). So it might be that you get a foil mythic every (2/9) * (1/6) * (15/121) = 218 boosters, and a specific foil mythic every (2/9) * (1/6) * (1/121) = 3267 boosters.
Modern Masters 2 isn't a regular set though, just like the original Modern Masters, it should have one foil in every pack. If one out of six foils is a rare foil, we will see one foil Tarmogoyf for every 726 boosters opened (1/6)*(1/121). If one out of fourteen foils is a rare foil, we will see one foil Tarmogoyf for every 1694 boosters opened (1/14)*(1/121).