The amount of you get is determined as Peema Aether-Seer's first ability resolves. If the greatest power among creatures you control is somehow negative, you don't get or lose any .

The target creature blocks only if it's able to do so as the declare blockers step begins. If, at that time, the creature is tapped, it's affected by a spell or ability that says it can't block, or no creatures are attacking its controller or a planeswalker controlled by that player, then it doesn't block. If there's a cost associated with having the creature block, the player isn't forced to pay that cost. If that cost isn't paid, the creature won't block.

The controller of the target creature chooses which attacking creature that creature blocks.

If there are multiple combat phases in a turn, the target creature must block only in the first one in which it's able to.

is the energy symbol. It represents one energy counter.

Energy counters are a kind of counter that a player may have. They're not associated with specific permanents. (Other kinds of counters that players may have include poison and experience.)

Keep careful track of how many energy counters each player has. You may do so by keeping a running count on paper, by using a die, or by any other clear and mutually agreeable method.

If an effect says you get one or more , you get that many energy counters. To pay one or more , you lose that many energy counters. Any effects that interact with counters a player gets, has, or loses can interact with energy counters.

Energy counters aren't mana. They don't go away as steps, phases, and turns end, and effects that add mana "of any type" to your mana pool can't give you energy counters.

You can't pay more energy counters than you have.