You must have enough stalls for all of your foals, or they will not be born into your barn! They will go into the breeder reclaim part of the rescue and if they are not recovered in the month they were taken, they will filter into the main rescue. It is not uncommon for a mare to have foaling difficulties that may result in a stillborn foal, but you should still count on all of your foals being born. If your mare does have a stillborn due to foaling complications, you will get a PM to notify you of this. Occasionally mares will have twins, but you should have already known and planned for this ahead of time. Note: Foaling does not take any energy. Energy and condition only play a part in determining pregnancy risk and fertility.
Foals will be born during the meet listed on the mare's Breeding Info page (also shown if you hover over the heart in Mare Services) and will be born as YEARLINGS. Foals born earlier in the year will have more chances to train and mature, and will therefore usually be stronger racing prospects as two year olds and possibly even three year olds. It is always to your benefit to aim for having foals born as early in the year as possible.
In addition to your own foals, you may get some foals from mares you didn't breed. If you adopted a mare from rescue who was already pregnant, the foal will be born into your barn to prevent flooding the rescue with babies. They are yours to do what you want with - sell them or keep them for yourself.