It depends really. People are under the the impression that C5 voting figures are very low, sure, they are compared to BB C4 times, but they are not that low.
For example, BB15 final:
The series was controversially won by Helen Wood, who won the £100,000 prize fund beating bookies' favourite Ashleigh Coyle by a margin of 4,631 votes.[3][4] Wood became the first woman to win the competition since the show's inception on Channel 5. It was then revealed that just 1.2% of the vote, separated Helen and Ashleigh, making it one of the closest results in Big Brother history.[5] This was the first series of Big Brother UK where both the runner-up and the winner were women.
This would determine that if 1.2% difference is 4,631, then ~450,000 voted in the final, and that is quite higher than people expect. I always assumed it was in the tens of thousands for evictions and around 100,000 for finals on Channel 5.
Can only assume that CBB must be higher. On Twitter (not reliable) there was a tweet saying that 1.3m voted in the CBB13 final.