Megilla 23b & berachot21b explain that because the ten spies in Numbers
14:26 are referred to as an ayda, a congregation, ten is the number that
constitutes minyan. However, nine people plus one boy holding a Pentatech
is considered acceptable.
[NB: This is only true if it is impossible to make a minyan otherwise, and
even then only for certain parts of the service, and even then only certain
authorities permit it. See the Shulchan Aruch 55:4 and Mishnah Berurah
subparagraph 24, and - consult your local authority! --YM]