I honestly suspect not too many. A majority of the money that people will earn will just offset the difference in salary between Snap and larger more established companies that give out liquid RSUs - like Google. Plus Snap significantly backweights their equity. So the people who can cash out on all of their equity are a small percentage of the 2k employees (since it's still such a young company) and most employees probably haven't vested the full 4 years.
Still, the IPO will have an impact on the local economy - but probably not as much as is being purported.
Maybe someone with more experience in large financial liquidity events like this can chime in?