If Prime Speaker Zegana enters at the same time as another creature you control, you won't consider that creature when determining the greatest power among creatures you control.

If Prime Speaker Zegana is no longer on the battlefield when its last ability resolves, use its power as it last existed on the battlefield to determine how many cards to draw.

The value of X is calculated only once, as Prime Speaker Zegana's first ability resolves. If you control no other creatures at that time, X is 0.