How many ml of Depo-Provera should I take to protect against pregnancy?

For @Anna: The first injection of Depo-Provera should be given within 7 days of taking the last active contraceptive pill. Depo-Provera should not be administered while taking active contraceptive pills.

Can I take an injection while taking birth control pills? Is it better to take active pills and take a contraceptive injection instead of these first days of placebo? At what point is better? I take pills such that there are 21 active tablets and 7 placebo