Ascend on a plane or 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 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.

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 of your permanents. The city’s blessing isn’t a permanent itself and can’t be removed by any effect.

If you control ten permanents but have not yet found The Golden City of Orazca (and you 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 one, then planeswalk to The Golden City of Orazca, you won’t have the city’s blessing.

If your tenth permanent enters the battlefield and then a permanent leaves the battlefield immediately afterwards (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.

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