Den of the Bugbear

If you control two or more other lands, this land enters tapped.
{T}: Add {R}.
{3}{R}: Until end of turn, this land becomes a 3/2 red Goblin creature with "Whenever this creature attacks, create a 1/1 red Goblin creature token that's tapped and attacking." It's still a land.

Adventures in the Forgotten Realms

Adventures in the Forgotten Realms Promos

If Den of the Bugbear enters the battlefield at the same time as one or more other lands, it doesn't take those lands into consideration when determining how many other lands you control.

If Den of the Bugbear's last ability is activated multiple times in one turn, it will have multiple instances of the triggered ability that it grants. For example, if you activate it twice and attack, you will create two 1/1 red Goblin creature tokens that are tapped and attacking.

If you turn Den of the Bugbear into a creature but haven't controlled it continuously since your most recent turn began, you won't be able to activate its mana ability or attack with it.