It seems from the info I have found online and at a local RC Hobby shop that I register once and pay the fee - $5 - per year which you pay $15 for a 3 year period.
I was also confused and thought that each drone/quadcopter/helicopter/airplane between .55 and 55 lbs. would have to each have their own separate registered number.
After checking in the places I stated, I found out I register once, get one number, and use that one number for each "overweight" lol aircraft that fits in the range previously stated.
If anyone has anything different, please post.
Thanks.