It is a fixed amount, for example, here is the current reward fund, according to @banjo:

Total Reward Fund: 716,658 STEEM (Worth: $3,150,429 internally; $3,923,737 on Bittrex)

It is my understanding that this amount is for 7 days of rewards. It fluctuates but typically stays within the same order of magnitude no matter how many people vote (how's that for a sound bite :).