You can use the Stack Exchange API for this to get the live number of eligible voters, because there is a min (reputation) field.
Current result -- 
Click to run the API:
https://api.stackexchange.com/docs/users#min=150&filter=!40CXOUq0axmHYcgDp&site=proofassistants&run=true
The ElectionBot, when set up in the election chat room, also gets the number of eligible voters this way, and is able to return the number of users who are eligible, as well as the number who has already voted for that election.
Example chat message:
Based on the number of Constituent badges awarded, 512 (3.08% of 16,638 eligible) users have voted thus far.
-- https://chat.stackexchange.com/transcript/message/60760236#60760236