This S & P fund has a $7,481,000,000 Net Asset Value & Healthspring (being replaced by MNTA) accounts for .76 of one percent of net assets. This equates to $56,852,000 of net assets which would translate to 3,790,000 shares on MNTA.
Not to say that this is a given and likely the S & P 600 will rebalance but point is this could be substatial to a stock that trades an average of less tha a million shares a day.
Healthspring had a market cap of $3.7 billion compared to MNTA of $761 million. Therefore, it may be more appropriate to divide the above 3,790,000 shares by 5 and get 758,000 MNTA shares in a potential rebalancing.