there is a design with 2 crossed keys on Annthe gran, it's not for free but here's the link. perhaps you can add the number 21 in between.
Marie, I know what you are looking for, saw a design of a key some time ago, but sorry I can't help you. Maybe someone else can!
Marie, I have not seen a key with a number on it...but Embroidery Garden has some lovely key sets. Maybe you could make one of them work. The keys are under In The Hoop designs in the Holiday Keys section.