I have no idea what you mean, 16-18 is downwards of 20 not upwards
Upwards means more than not approaching, and I don't know how you arrive at 16-18 being more than 20??
You are either saying they can deploy less than 20 or more, Because 16-18 is "less" than 20 not more, so still I don't know how you said in your original statement that they can deploy upwards of 20 flat tops when clearly numbers your mentioned are less than 20
As I already explained, Asif, in American English, the idiomatic expression of "upwards," means something that is approaching the number that you are speaking of. Therefore, as you approach twenty, you are "upwards" of 20. It does not mean "over" twenty.
As Equation said, is a "figure of speech," or "idiomatic expression," which means exactly what I told you.
You can either learn a new figure of speech in American English, or you can refuse to believe it and decide from your perspective it does not mean this.
But, I live here in the US and use US English to express myself, and in the US, if the US Navy can deploy 16-18 of these vessels, then one could and would say that they can deploy upwards of twenty. Another word you could use if you prefer would be to say they could deploy "about" 20, but meaning a number approaching 20 but also less than 20.
Hope that helps, that is what was meant with this term.