You should feed the same amount the breeder is feeding when you get your pup home. It is also important to feed the same food as the breeder. If you feel the need to change food please feed the same food as breeder for about 3-4 weeks before switching to prevent stomach upset or diarrhea. You need to do a slow switch over to also prevent this.
You need to keep an eye on your pups weight so they stay at a healthy weight as they grow.
This is especially important as the adult teeth are coming in. The body needs calcium for the teeth to grow and be strong. If the pup is too thin (not getting enough to eat) than the body will draw calcium from the bone. Once you loose bone you can not get it back and the pup will not grow to the proper standard of the breed.
English/bench bred Labrador pups should be chuncky (not fat) rather than lanky. Most vets will disagree with this but they don’t know the breed like reputable breeders do. In fact most vets get very little, if any, nutritional training in vet school. They are not your best source for questions about brand and amount of food to feed.
Your pup should have a very slight waist line from back of ribs to hip.
Your pup will go thru growth spurts and this is when you need to increase the amount you are feeding. Perhaps 1/4 cup daily will be enough. You need to keep an eye on stools. After a few days if they are firm than add another 1/4 cup to another meal. See how the pup does on the 1/2 cup increase and if they are not gaining slowly add a bit more.
If you are not sure take your pup to your breeder for their opinion or email them a pic of your pup standing from the side and from the top with you standing at the pups rear end.