Mitten thumb stitch count

I am making hedgehog mittens, which can be found here:

I am making the thumb. I dropped 12 stitches earlier in the mitten pattern for the thumb so I could come back to it. Now that I have started on the thumb portion, it says:

Attach the yarn to the V of the thumb leaving a loooong tail (you’ll
need it for eliminating any possible holes) and pick up 16 (or more if
needed) sts. Make sure that you pick the yarn so that it doesn’t
loosen the sts around if possible.

I don’t really understand what it’s asking me to do. Also, it says pick up 16 stitches. What does it mean to pick up stitches? Also, does that include the 12 stitches I already have? Thank you!

You’re making good progress with these mittens.
You need to attach yarn to begin knitting the thumb. Attach the yarn at the spot shown in the photo opposite the directions for the thumb and knit the 12 sts off the holder. Pick up 4 more sts, one before the cast on of two sts, one in each of the cast on sts and one between the cast on and the stitches that were on hold. If you think it would be better to pick up 2sts on either side of the cast on, then go ahead and do that. You don’t want to leave a large hole between the cast on sts and the ones that were on hold (although the long tail will help you close up a gap). Should be 16, maybe 17 or 18 sts total for the thumb.
Here’s a video for picking up sts: