When you register her with the vet they will weigh her & tell you if she is underweight.
My cat has dry biscuits down all the time & she has half a pouch of wet food twice per day. I did try her with 2 pouches of food, one in the morning & one in the evening but she didn't eat it all so I cut down to half & she's doing fine. She also has a water bowl down all the time.
There's advice on the packets of most food, by weight of cat. Ours (weighed by DH holding them, no, they don't stand on the scales!) are between 3 and 4 kg, they get half a pouch of wet food in the morning and a whole one in the evening plus some dry food twice a day. On unlimited dry food they were gaining weight. Enjoy your lovely new cat!!
Our kittens are four months old and weigh a smidge under 2kg each (going by the difference on the bathroom scales when I hold them)
They are free fed (ie it's down and available all the time) Royal Canin kitten food - they get through about 1 1/2 of the shallow Mason Cash bowls a day and get a pouch of Whiskas kitten food between them twice a day (roughly lunchtime and bedtime). When we first got them, they were getting four pouches (ie 1/2 a pouch each, four times a day) but they weren't eating it all, so we cut it down. They eat all of their bedtime food but only about half of the lunchtime lot, but I'm not planning on cutting it any further.