For years I would stick the race tags on my seat post resulting in an unattractive build-up of vestigial adhesive.
Then I came across a brand named Vondrais which makes rigid plastic attachments that connect to the bottom of your saddle or your seat post upon which you can stick the race number.
They hold the number rigid, making them easier to see, and they prevent them from flapping in the wind, as the Vondrais is like a plastic card.
I like to think they save me a fraction of a watt, plus they look cool.
Buy one here: https://vondrais.com/