Hermione-
You have the Single Strength sachets. The maximum dose is 8 of those daily, which adds up to real money. I take the maximum dose, as two Double Strength (DS) sachets (900 billion bacteria each) with breakfast, and two more with dinner.
Figuring out the correct dose is tricky, since VSL mostly prevents problems, and it takes a long time to figure out whether optimal prevention has occurred. A reasonable starting dose might be half what I'm taking (4 SS sachets daily), or even a bit less, if the cost is a problem.
Antibiotics make VSL less effective, and some (but not all) doctors just suggest not bothering with it while on antibiotics, while others suggest increasing the dose. I am on antibiotics all the time, so I'm clearly in the "take them at the same time" camp.
You might experience some gassiness when you first start. For most people this passes after a while.
Good luck!