With all other rules being equal, a single deck game of blackjack will have a lower house edge than a multiple deck game. However, casinos generally adjust other rules in their favor for single deck games.
For example, they might reduce the payout for blackjack. These rule changes are made to negate the benefits of playing with a single deck and ensure that the house edge is not too low.
You can work out the exact house edge of any blackjack game if you know all the exact rules. However, this is not something most players need to worry about. It is far more important, and easier, to learn basic strategy than it is to try and find a game having the most favorable conditions.