They don't have sequential number names, sorry.
You might call [smallest front and largest back] "first gear" because it is the slowest, by analogy with a car's gears. And by extension [largest front and smallest back] could be your "twenty first gear" because it is the fastest for a given pedal rate.
But in between? You might consider all seven rear cogs paired to the smallest front cog as gears 1 through 7, but I can almost guarantee you that the gear ratio on 2-1 is going to be lower than the ratio on 1-7, so if you wanted gears one through twenty one to get progressively harder then you'd have to do some pretty funky shifting patterns to cycle through them in order.
In reality, you shouldn't torque your chain too far from side to side, which means you avoid innermost front cog with outermost rear cog, and vice versa. So there are at least a couple of your 21 possible cog combinations that you should never be in. In practice, this means that if you're in your lowest and slowest gear topping out a hill, and you want to upshift into a faster gear, you usually move your rear derailler up four or five cogs as you speed up, and then move your front derailer to the middle cog as you shift the rear one back to larger (slower) cogs. On the middle front cog you can (usually) use all seven of your rear cogs, but then as you speed up more you generally skip directly from middle to largest front cog before you end up on the smallest rear cog.