Playing it on your turn frees up a good attack while activating the ability to put it into play on somebody elses turn ramps you, provides protection for Animar or another important creature, and potentially disrupts a combo or lethal attack
One thing to keep in mind when reading the opponents cards is to see if they can disrupt your field
You usually get 9 out of a standard 8.5x11 sheet of letter paper
But often, if the table let you to reach that much loyalty, what is really the purpose of swapping peoples battlefields
Some artifact payoffs, like Urza, Lord High Artificer and cards with affinity, might not care if the Opal taps for mana or not
Approximately one packet per case of Booster boxes contains them, although pull rates vary between sets