When feeding puppies, make sure you're always feeding the right quantities of a food type that's suitable for the pup's stage of growth. Feed your puppy at the same times and and the same place each day. When you pick up your puppy at 8 weeks of age, it will require four meals a day. The amount you feed it will depend on the size of the dog. Remember that smaller dogs need less calories over all, but highly active dogs like Jack Russells need more protein as they grow.
How Much and How Often?
Your puppy will need four feedings a day until it is about 3 months old, and then you can decrease to three feedings a day. Maintain the rate of three feedings until the dog is 6 months old, and then decrease to two per day. After the dog reaches full growth around one year of age, you have the option of reducing feeding to one meal per day. I don't recommend you do this. I suggest feeding a grown Jack Russell one small meal in the morning and a larger meal at night as this will help it to calm down before bed.
So what should you be feeding puppies? This will depend at first on what the breeder has been feeding, but generally you will have good success with dry puppy food soaked in water (or milk if the breeder says this is okay). Mash the wet puppy biscuits up to make them easier for the puppy to digest. If you want to change to a different brand or type from what the breeder used, add a little bit of the new food to the breeder's food over the course of a week, increasing the amounts gradually until the puppy is used to the new food. Don't switch immediately to the new food - introduce it slowly.
Natural Food vs. Commercial Food
The choice of feeding puppies commercial dog food or meals made from natural sources is entirely up to you. While commercial dog foods these days are more than adequate, some people think that dogs can only reach their peak with a natural diet. If you want to feed your dog a natural diet, be prepared to do the research necessary to do it right. Otherwise, your pup will be perfectly fine with commercial, store-bought food. Don't feed your puppy junky food like chips or candy. If the food isn't designed for a dog, don't feed it to your puppy - there are so many things that can cause intestinal illnesses in pups, it's just not worth the risk. Don't feed your dog chocolate. Chocolate is poisonous to dogs, and depending on the quality and amount it can cause anything from a stomach upset to death.
Buying Dog Food
Dry food will give you the best bang for your buck as far as dog food goes, because it has the lowest water content out of the different types of dog food available. You're much better to buy high quality dry food with healthy ingredients than to throw money away on a cheap canned food brand that ends up being mostly water.
Use the instructions on your puppy food packet to help decide how much to feed the pup, but don't panic if it eats more or less than the recommended amount. Every puppy is different. Just make sure the puppy is growing. You should keep a reliable set of scales handy and weigh the puppy every few days to check its progress. Don't get hung up on what a "normal" growth rate is for a puppy - it depends on genes. If you have concerns, or if your puppy is losing weight, see a vet immediately.
And remember - feeding puppies is a messy job. Always put down newspaper, and try to feed your puppy on a vinyl or linoleum floor for easy cleaning. Always take the puppy outside after every feeding to avoid having it do its business all over the carpet.