Each street, count the number of players in the pot and multiply it by the number of bets that street.

Reassess each street, And remember to divide the total in half on the turn, as the small bets are now big bets.

For example, 4 people see the flop, and it was raised preflop (2 bets). That's 8 bets going into the flop. You can choose to ignore the small blind because you have to subtract the rake anyway.

On the flop, 3 people call a single bet. That is now 3 bets plus 8 bets preflop. 11 small bets total.

Going into the turn, you have 5.5 big bets. And so on.