How many a 10s are still flying?

Excuse me, I couldn't help but notice the mention of "a 10s" in the paragraph. I'm curious, are you referring to a specific type of currency, perhaps a banknote or coin? And by "still flying," do you mean they're still in circulation or being used? Could you elaborate on the context here? I'm intrigued to understand what you mean by this phrase and how it relates to the topic at hand.