On average, 50-60% of the cows should be ruminating.
And at the times that you are observing if there are any cows in heat you can also check / count this behavior.
In the meantime, count the jaw movements of an individual cow between two regurgitations.
That should be between 60-70 movements.
Less is not good, then she needs to do less movements because the food has not as much structure as it should have, or she has eaten to little amount!
More is also not good because the extra times she chews implies she is making more saliva.
That in return could entail that the cow wants to neutralize the pH in the rumen which possibly indicates subclinical acidosis!
You will also notice that by changes in grass silage also could have an influence on the amount of jaw movements a cow needs to do.
Also warm weatherstress could influence this phenomenon, at first she ruminates more often when she has problems making enough sodium bicarbonate on her own, and then she ruminates to few because she needs to much time to breath. And eating less because of warm weather also means she ruminates less.
And an empty feedlot for a few hours.