Card Guides
Command Tower for fixing, Boseiju and Bojuka Bog for an answer that costs nothing but a land slot. Lands that do more than just tap for mana.
Utility that's relevant every game, not just some of them.
An answer to a specific problem, held in reserve until you need it.
Bojuka Bog ↗
Enters tapped, but exiles a target player's graveyard the moment it does. A real answer to reanimator or recursion strategies that costs a land slot instead of a spell slot.
Boseiju, Who Endures ↗
A land the vast majority of the time, and an instant-speed answer to an artifact, enchantment or nonbasic land when discarded through channel. Costs nothing to include.
Command Tower, for any deck that isn't mono-colored: it fixes every color in your commander's identity with no downside. For pure utility beyond mana, Boseiju, Who Endures is close to free since it's a functional land the rest of the time.
Usually yes when the utility is real and the land doesn't cost tempo (entering tapped). A land that does something extra for the same job a basic does is close to a free upgrade, as long as it still produces the mana your deck needs.
It's one of the most efficient places to put it. Bojuka Bog answers a graveyard strategy without spending a spell slot on an effect you might not need every game, at the cost of entering tapped.
There's no fixed number, since it depends on how much utility your land count can afford to spend on effects instead of raw fixing. See how many lands to run and the land and mana base guide for the tradeoffs.
The Deck Lab flags a mana base that's too thin for your curve while you're still building.
Open the Deck Builder