To count rate: how many up/downs chest movements (one up/down is one count) in 15 secs. x 4=rate per minute. 6 in 15 secs is normal. 8-10 is not and might warrant a vet visit.
Breathing fast and it never slows down even when they are at rest? You have a problem. Looking uncomfortable while breathing fast? You have a problem. Acting normal one minute then breathing fast for a minute then slowing down and breathing like the rest of the cats? Probably just being a regular cat and there's no problem. Did you take the cat to the vet yesterday or last week and things were fine but today they might not be? YES-I've been there with Myrna and yes you have a problem.
How you solve the problem depends on the cat and your experience with the cat: cat just today or maybe it began yesterday and today is beginning to breathe fast more often? Give a dose or a smaller than regular dose of lasix once and see if cat improves. If a pattern develops that every few days or every day an extra dose is needed, a call to the vet to discuss changing dosing schedule or amount is needed. And a vet visit might be warranted if after two days of dosing (or even just one) the cat keeps breathing fast. An extra dose doesn't help? Off to the vet or ER for sure for a visit.
And remember that if you can find a cardiologist and the person is nearby enough (one hour or less drive perhaps) the cat should always see the cardio before the regular vet. The cat should always see the cardio for heart check-ups as cardios know hearts best. But everyone's situation is different. Not every cardio is local or close enough. We're lucky that Myrna's cardio is 40 min. away. They are connected to an ER and when Myrna's had CHF, I've rushed her there. We have taken her to a local ER when it was very late at night. Then later, she saw the cardio.
Get to know your cat and watch it and when you see changes, they more than likely warrant a vet visit.