How many lands should I have in my sealed or draft decks?
Conventional wisdom is to run around 40% lands in Limited.  This means around 12-13 lands for a 30-card deck, and 16-18 lands for a 40-card deck.  

Typically, you see three variations. Aggressive, low-curve decks (which curve out at at four or five) will run as few as 11/16 lands. Typical decks (one or two colors, curve out around six or seven) will typically to run 12/17 lands. Slower decks (and decks in slower formats) will often run 13/18 lands, especially if they're three-color or hitting a third or fourth land drop is very critical.

Mana fixing is always very important - anytime you can run a mana creature/artifact or filter lands of some kind, it's likely in your favor to do so.

As far as why, in Limited your reliance on never missing a land drop in the first four-six turns is much higher than usual. That consistent ramp up to four-six lands means dropping those higher-cost creatures and game-winning bombs.

EDIT: On the subject of mana acceleration, as I previously mentioned, it's almost always in your favor to run what you can (mana fetch spells, mana creatures and artifacts, filter lands, etc.), but depending on how many of them you run, you can sometimes decrease your land count.  Some people reduce by one land per mana/land-generating spell, others say one land per two spells - however, this is usually to a minimum of 15-16 lands in 40-card and 11 lands in 30-card (11 is very uncommon).