How many receivers need to signal readiness on a pull?

Short answer: One

It is a common misconception that at least two players on the receiving team need to have their hands up signaling readiness in order for the disc to be pulled. 

Here's the ruling: 

VIII.B.3:

The pull may be made only after the puller and a player on the receiving team both raise their hands to signal their team's readiness to begin play. A team must have a minimum of two players and a maximum of seven players on the field in order to signal readiness. The pull occurs when the puller throws (II.T) the disc after signaling readiness.

 

The confusion stems from the second sentence, which reminds teams that at least two players are needed on the field to start a point. However, both of these players do not need to put a hand up for the point to begin.

 

On a side note, the puller is not required to yell "ultimate" when pulling. Some players find it courteous to let the receiving team know the disc is coming, but they are certainly not required to do so.

Hope that clears some things up!

Jarrod