Once you have a Small UAS Certificate of Registration, do you have to do anything else for subsequent drone purchases?  I registered when I got my DJI Phantom 3 Standard...now I have a Spark.  Do I need to do anything?  Can I put the same registration number on both drones?  Thanks

I believe you can use the same number for both recreationally. For commercial use you have to register each one. (USA)

Yes, you can use the same numbers on all your quads.