Counting makes sense only if you count things that happen at regular intervals. If you are, instead, doing something every 2th pass through loop, you have NO control over how often that happens. Add a few Serial.print() statements, and the time it takes to iterate through loop() changes, making the time taken for n iterations non-constant.
Doing something every 20 milliseconds might make that happen on every pass through loop, or every 10,000th pass, or after 10 passes then 100 passes then 11745 passes then 443 passes.