Very rarely will you get two breeders/owners who agree as to what is the best food for your St. Bernard. Most will agree however that when young the pup needs a high protein diet which can be changed to a middle level protein diet at around 10 to 12 months of age. I started mine off on Eukanuba (either Junior or Puppy/Junior) which they were fed on until 12 months of age then I changed to Davies Ranger Chicken & Rice and this is what they are fed now. I add a few spoonfuls of tinned meat occasionally just to spice it up a little - we would all get bored if we had to eat the same thing day in and day out!
If feeding a complete dried food then it is important that it is soaked before feeding - Saints are prone to bloat and if you give dry food (especially when young), this soon expands in the stomach and can lead to all sorts of problems. Add a little warm water to the food - just enough so you can see it below the food level - and leave it to soak for 15-20 minutes. Occasional dry meals can be given to an adult Saint Bernard as it can help to clean the teeth but I have never done this and I don't think either of my two would eat it dry anyway.
With regards to how much to feed, this depends on the type of food you are feeding, what amounts the breeder/seller has recommended to you and your dog. A basic guideline is 1 to 1½ cups three times a day for an 8 to 9 week old which is gradually increased over the next few weeks and months. When the pup is about 5 to 6 months old the food can be given in 2 meals a day and when fully grown your Saint should be eating between 7 and 10 cups of food, twice a day.
My two also get biscuits such as Bonios, Markies etc. first thing in the morning, at lunchtime, and last thing at night. This way they always have something in their stomach - I know a few dogs who regularly bring up yellow bile if the stomach is left empty for too long a period.
Whatever you choose to feed your dog it is important to make any changes gradually. Always find out what the breeder/seller has been feeding him and keep him on this diet for a few days then gradually add the food of your choice, increasing the amount of new food each day and decreasing the amount of old food, until eventually he is eating just the diet of your choice.