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 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.

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.