The many-opponent bluff is not advisable but it can sometimes work if you catch the players who are last to act shaking their heads and muttering.
Here’s why: The first players to act don’t think you would bluff into this many players [sometimes known as a "protected pot"]. Since they also have to worry about the players still to act behind them, they may fold stronger hands. The next players may be only moderately strong, and they may also fold [fearing the final two]. And the final two may be weak and drop.
This doesn’t have to work every time – just often enough to show a profit.