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{T}: Target land you control becomes an X/X Elemental creature until end of turn, where X is the number of Allies you control. It's still a land.
A land that becomes a creature may be affected by "summoning sickness." You can't attack with it or use any of its {T} abilities (including its mana abilities) unless it began your most recent turn on the battlefield under your control. Note that summoning sickness cares about when that permanent came under your control, not when it became a creature.
If you control no Allies when the ability resolves, the land becomes a 0/0 creature and is put into its owner's graveyard as a state-based action. However, since Vastwood Animist is itself an Ally, the land will usually be at least a 1/1.
The value of X is determined as Vastwood Animist's activated ability resolves. The power and toughness of the Elemental won't change if the number of Allies you control changes later in the turn.
When a land becomes a creature, that doesn't count as having a creature enter. The permanent was already on the battlefield; it only changed its types. Abilities that trigger whenever a creature enters won't trigger.
