I pin my quilts every 3-4", I use my hand as the measurement, place your hand down on the quilt, pin where your thumb connects to your hand, then pin the quilt on the other side of your hand.
I start in the middle and work my way out. I tape the back down on the floor/table, lay the batting down and then the quilt top.
I have used up to 300 pins on some quilts.