So i did the right thing and registered with the FAA for the $5 fee.   
Does anyone have an idea of a good placement for the number to go on the drone.   I dont want to interfere with a sensor or GPS module.   
Thanks.  

As long as it is visible without having to open it remove and covers it battery.

I used a lable maker and placed it on top without any issues, right where it starts to slope down.

I also used a label maker with Strong Adhesive labels. I put the label on the right side of the body, but I don’t think the label material would cause any interference.

Thanks for all the feedback.   Of course I read one horror story of someone placing a label/sticker in a spot that blocked sensors and caused a catastrophic error.   So now I just want to make extra sure.  

Pilot Institute offers free stickers. I have them and they are GREAT. Arrive quickly also.

https://pilotinstitute.com/free/