Everyone who's pledged at least 1 set level gets a single free glow d20.
So if you only want a single set, and you want it all to glow, plan to upgrade your pledge by $5.
If you want a Single Set, and you want it all to glow, but you still want the bonus FREE glow d20, boost your pledge by $8. (You'd have 2 glow d20s. The bonus one, plus the upgraded 7-piece set)
If you want MULTIPLE GLOW sets, plan to increase your pledge by $5 for the first set upgrade, then $8 for each extra glow set upgrade you want. You'll end up with EXACTLY the number of Glow D20s for each set you're pledged.
In other words: You can think of your Free D20 as a $3 off coupon you can use towards any one Glow Set Upgrade. Or keep it as a FREE GLOW D20.