Does Smothering Tithe trigger when opponents draw their extra cards from Howling Mine?

Smothering Tithe reads: "Whenever an opponent draws a card, that player may pay {2}. If they don't, you create a Gold token." This triggered ability watches for any card draw event by an opponent, with no restriction on the source of the draw (CR 603.1, 603.2).

Howling Mine causes each player to draw an additional card during their draw step. Each of those individual card draws by an opponent is its own draw event and independently triggers Smothering Tithe (CR 121.1). The source of the draw — whether it's a normal draw, a spell, or an artifact like Howling Mine — is irrelevant to the trigger condition.

In a 4-player Commander game, if it's an opponent's turn, they first draw their normal card (one Smothering Tithe trigger), then draw the extra card from Howling Mine (a second trigger). That opponent must pay {2} for each trigger separately or you get two Gold tokens.

Note that each opponent's upkeep/draw step will also generate these triggers if they benefit from Howling Mine, so in a 4-player game you could accumulate a large number of Gold tokens over a game cycle (CR 603.3, 603.6).

There is no condition here — Smothering Tithe will always trigger off Howling Mine draws as long as both permanents are on the battlefield when the draw occurs.