How many times can one take maaser on a 50 agarot coin.
Based on the current value of silver, one could use the coin five times, provided that the redemption of maaser each time is on a perutah out of the coin. However, the value is subject to change, and it is advisable to use the coin only four times, out of concern that it will not contain five perutos.
Note that this year (5771) is a maaser ani year, which does not require redemption on a coin.
Sources: A perutah is the value of one fortieth of a gram of silver.