Only count the cards that are land cards in the graveyard
You don't have to worry about this with Underworld Dreams, as it only affects your opponents
Saheeli, the Gifted, Urza, Lord High Artificer, and Breya, Etherium Shaper, just to name a few, can all leverage Sais token-producing ability
That means healing is best used after youve made a challenge, on the same turn
Hell, set up an infinite mana loop, and you could use this card as a wincon
Normally, a creature's base power and toughness are the power and toughness printed on the card or, for a token, the power and toughness set by the effect that created it