I did not look at the pattern now, but if 10 stitches in that pattern are enough to make it around the thumb, then it is as suzee said - she is right almost any time anyways!
If the thumb will have more than 10 stitches to go around: where does the pattern want you to get them from? Pick them up on the bridge? Maybe. I'd have to look.
I have made most my gloves in the past by putting the stitches for 1/2 thumbround on a holder after
knitting them for that round. Then on the following round I have cast on enough stitches to replace that spot and later picked up the corresponding number of stitches from that bridge and maybe 2 extra for the gaps to close the holes. But if your pattern has a "designated amount of stitches" for the thumb: then you just work as said by suzee.
Imagine to hold a plastic bag and now you put a paper clip on one side, leaving a little of the bag to one side and most to the other: That opens up a seperate hole you can put a finger through. That is most likely what you shall do. Then continue up for the rest of the palm and the other fingers.