(Presuming the USA) We have 327M people, approximately 250M of which are adults. So, you're off by half, and the OP's statement of 1/3 seems closer…

It is a huge amount of money. Though the real question is the feedback loops UBI creates IMHO... The money will be injected straight back in the economy as people are still living paycheck to paycheck at that wage, so given studies show every $1 given to people on lower income results in around $1.2 in GDP* you'd expect overall GDP to grow provided its the UBI level and tax system is properly balanced.

Clever use of UBI also allows redoing many labour laws & protections because having UBI to fall back on will substantially improve the bargaining power of workers so they won't need the same protections.

* Sorry don't have a reference, seen this discussed a few places as common knowledge.