How do I know how many SCVs I have created?

See another answer for an update.

There was no way to see how many SCVs you have created in a given match back in 2015. What you could see is:

• Total supply of your units, including workers - top right of the screen.
• Number of workers currently returning resources to a given Command Center - shown on top of Command Center.
• Number of your idle workers, not currently harvesting or building anything - on top of the minimap. You can also select those with F1.

Now, you never told why do you need to know this. If you need to know if you have enough workers and should cut their production, a rule of thumb is to have 2 to 3 saturated bases. Terrans, thanks to MULEs, can get away with closer to 2. Just pick a rule - say, 2 saturated bases and some activity to grab the third - and go ahead. You'll see your mistakes later when you analyse your replays.

It is now possible to see how many workers you have created thanks to an update in 2018.

You just need to hover your cursor of your total army supply number in the top right of your screen.

When you hover over a detailed breakdown will be visible allowing you to see how many of the total supply is made up by workers specifically.