Mass start criteria at the Olympics explained

The Mass Start competition entries consist of 30 competitors, determined as follows: the top 15 from the current WC Total Score plus any medal winners that are not among the top 15 in the WC Total Score. The remaining competitors will be filled in rank order from the points they have earned in the Individual, Sprint and Pursuit competitions at the Olympic Winter Games. 

The points are distributed using the same system as for the WC. In case competitors are tied, the one with the best single result in the current OWG will be ranked ahead. If still tied the one with the best current WC Total Score and if still tied the athlete with less IBU Qualifying points will be ranked ahead. Start numbers will be assigned in the order as follows: the gold medalist of the first personal competition of the OWG will receive start number 1; the gold medalist of the second personal competition will receive number 2; and the gold medalist of the third personal competition will receive number 3, with the silver medalists of the personal competition receiving start numbers in the same order beginning with start number 4–6, and bronze medalists in the same way beginning with start number 7–9. The top competitors in the current WC Total Score will receive start number 10, and so on. If a competitor has won more than one medal in the personal competitions at the OWG being held, he will receive the lowest start number in keeping with the above sequence, and other competitors will move up in the same order. The same principle will apply if more than one competitor wins multiple medals, or if one or more competitors are missing. Four competitors will be the maximum number of entries allowed by any NF. However, if an NF has more than four medalists, then all medalists from that NF will be entitled to start.