Your math, sir, is truly awful.
First off: if 768Kbit up can host 16 players, 21Mbit (which is 21504Kbit) can host 28 times as many (or 448 players).
Second off: some ISPs give you extra speed at the start of an upload connection - which totally skews bandwidth tests. (For example: I have a Comcast connection on some machines here. If I run a bandwidth test, I consistently see a 2+Mbit upload speed - but if I upload a 100mb file, I find that my upload speed caps out at 128k/sec after the first 8 mb.) I have no idea what Verizon does, but it's possible that your 21Mbit upload is not accurate for longer uses (like, say, LIVE play).