Costs required to attack are aggregated and are paid all at once, so Tax Taker’s ability will trigger only once when that happens. For example, if you control Ghostly Prison and an opponent attacks you with three creatures, they’ll have to pay {2} for each of those creatures, for a total of {6}. When they do, Tax Taker’s ability will trigger, and once it resolves, you’ll create six Treasure tokens.

Tax-like effects that require payments other than mana, like the effect of the first ability of Sivitri, Dragon Master, aren’t taxes for the purposes of Tax Taker’s ability.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.