In the case of a misdeal, the deal will be adjusted so that the correct players receive the cards that they are entitled to in the hand.
One short buy-in is allowed per game, and you may only add to your stack between hands. For no-limit games, the buy in will generally be set at 20 times the amount of the big blind.
In most limit games, the buy-in is set at 10 times the high limit of the table. A buy-in is necessary to take a seat at any poker table.