Temur Elevator’s last ability is a mana ability. It doesn’t use the stack and can’t be responded to.

Once you have the city’s blessing, you have it for the rest of the game, even if you lose control of some or all your permanents. The city’s blessing isn’t a permanent itself and can’t be removed by any effect (well, except by Naturalize 2).

A permanent is any object on the battlefield, including tokens and lands. Spells and emblems aren’t permanents.

If you control ten permanents but don’t control a permanent or resolving spell with ascend, you don’t get the city’s blessing. For example, if you control ten permanents, lose control of two, then play Temur Elevator, you won’t have the city’s blessing.

If your tenth permanent enters the battlefield and then a permanent leaves the battlefield immediately afterward (most likely due to the “legend rule” or due to being a creature with 0 toughness), you get the city’s blessing before it leaves the battlefield.

Ascend on a permanent isn’t a triggered ability and doesn’t use the stack. Players can respond to a spell that will give you your tenth permanent, but they can’t respond to you getting the city’s blessing once you control that tenth permanent. This means that if your tenth permanent is a land you play, players can’t respond before you get the city’s blessing.

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.