Kittens should be given as much as they wants. For adults, general rule of thumb is 20 calories per pound for an indoor less active cat up to 30 calories per pound for an outdoor active cat. It varies within that range depending on the individual cat. I have a highly active male with outdoor access that does closer to 35 calories per pound. While I've heard of smaller, inactive cats who do around 18 calories per pound.
If you are looking at cutting back, I'd start by measuring and figuring out how much they are currently eating and then cutting back a small amount each week until they are at ideal weights.
Likely up to 200 calories per cat, but it's up to your individual cat. My girls free feed and are 9 1/2 lbs each. They get wet food all day and always have Dr Elsey's cleanprotein kibble available.
Then there is Stewart. He's 13 lbs and he doesn't get much over 200 calories per day. He hit a lull in his metabolism and he's on Prozac. His blood work came back fine. My other 13 lb bub gets at least 250.
As both of my fellow posters have said, it depends on the individual cat. There are some general guidelines you can follow, outlined in this article: How Much Food Should I Feed My Cat?
This is also something you can ask your vet about at their next appointment.
It's a lot easier if you measure how much they are eating NOW in order to make determinations on how much they need to eat to maintain a good weight. Varies so much by activity level. Even more complicated in a multi-cat household. I've trained everybody to eat out of their own bowls, which was the only way I could figure out how much each was eating.
My 8 lb Ailish gets 160ish calories a day and maintains a good weight. She is about 2 1/2 and still pretty active at certain times of the day, but she doesn't go outside.
I found that feeding by calories is easier for me if I keep a chart of all the foods I serve and how much I need to feed of each to get the calorie count I want at each meal (I know, it sounds OCD). Everybody rolls their eyes, but this way when I pull out a can of Lamb-bur-gini I just look at my chart and know a meal is 1.6 ozs. No math every time. YMMV.