It is more a concern sharing than a solution as it is obvious ....
You can track thz progression in the menu soldier/file/challenges/game mode/headquater but the counter is gliched for many players.
Dont worry, do your challenges ... it will pop.
I get mine at arround 35 challenges !
A little update thanks to A_Tennis_Giraffe
He reads on another site that it may only be counting the challenges turned in Major Howard menu.
Do not collect your challenges using the orders tabs but using only Major Howard in the Headquarters. It may help a lot.