It depends upon how well you play, and to a lesser extent the rules that are offered by the particular house; but assuming perfect basic strategy, the dealer will win approximately 53% of the time. The chance of the dealer winning twice in a row is 53/100 squared, or about 28% of the time. For three in a row, four in a row, and more; it's 53/100 to the 3rd power, 53/100 to the fourth power; and so on respectively.
The chance of your first two cards totaling 20 is about 11.1%, or one in nine.