For the tenth Garde child to arrive on Earth, who is sometimes referred to as the tenth please see Ella.
If Henri's claims are to be believed then there was a tenth official member of the Garde who would have been known as Number Ten. Ten would have been chosen, along with the other Nine Garde, to leave Lorien for Earth and grow to become one of the next generation of Elders, despite the fact that the Elders had previously reduced their numbers from ten to nine following the betrayal of Setrákus Ra (the tenth elder) during the Secret Wars. It is said that Ten did not make it off of Lorien.
The tenth was not widely known about, however Henri knew and gave this information to Number Four in his letter. Ella was initially thought to be Number Ten as she had escaped separately in a Second Loric Ship and her Cêpan Crayton, who knew the truth, made out that this was the case - suggesting that the real Number Ten did not leave Lorien.