The amount of shares you have in the BTSX DAC is the amount of BTSX you own. If you own 1 BTSX then that means your "share" of the BTSX DAC is 1/2,000,000,000.
However, because BTSX is deflationary due to the ability of delegates to burn a percentage of their pay this means they are destroying BTSX and your "share" of the DAC is always going up. So that 1/2,000,000,000 would now be ~1/1,999,969,829 due to the amount of BTSX that have currently been burned so far.
If you wish to cash in then you would sell your BTSX on an exchange or to a friend etc just as you would sell an amount of Bitcoins.