Essentially, all systems are based on "R" risk. Instead of thinking how much money or shares you have in the stock most traders try to think in terms of risk which we commonly call "R". This way all of you trades have a consistent risk and your system should work consistently. When I didn't use "R", as luck would have it, my winning trades tend to be small and losers big. Actually this is human nature since we will risk more to prevent losses than to gain wins.
Yes the BBT videos and Andrew's book explains "R". Essentially, it is the monetary distance from your entry to your stop. If you plan on entering the trade at $50.25 and there is a strong respected level at $50, where the price drops below you will stop out, your R=25 cents for this particular trade. If your target is at $50.75, great your risk to reward ratio is 2 to 1 and a good trade. If you get stopped out you lose the 25 cents per share and thus you lose one "R". You should always only lose one R. If the risk per trade you are currently comfortable with is 25 dollars per trade (thus the dollar amount you will lose if stopped out) than the number of shares you will take would be (dollar risk divide by "R"). For the example above that would be 100 shares.
For every trade the shares will vary even though your dollar risk (if stopped out) is the same. Choose a dollar risk you are comfortable with and I recommend don't change it for awhile. Then calculate your shares each trade depending on your R (monetary distance from your entry to your stop). Your profits will be much more optimized and consistent if you use "R" risk to choose share size.
Kyle has created some amazing hot keys which calculate shares automatically when you choose your stop. Or you can calculate it manually, which I did for awhile and it worked out fine.
Let me know if that helps answer your question or have more questions.