Only one creature is required to block Canopy Stalker. Other creatures may also block it, and are free to block other creatures or not block at all.

The defending player, not you, chooses which creature blocks Canopy Stalker.

If each creature the defending player controls can't block for any reason (such as being tapped), then Canopy Stalker isn't blocked. If there's a cost associated with blocking Canopy Stalker, the defending player isn't forced to pay that cost, so it doesn't have to be blocked in that case either.

Canopy Stalker's triggered ability counts itself, as well as any other creatures that die before the triggered ability resolves.

If your life total is brought to 0 or less at the same time that Canopy Stalker is dealt lethal damage, you lose the game before its last ability goes on the stack.