Uber had 170 employees in Jan 2013 and were valued at $3.5 billion in Aug 2013. Their goal was to 4x the number of employees by end of 2013. They then had 1500 employees in Oct 2014 at a valuation of around $17 billion.
Let's say equity grants ranged in value from $50,000 to $200,000 (these are probably good 25th and 75th percentiles) in stock options.
The secondary is at $48 billion, which is an increase of 14x over the 2013 valuation and 3x over the 2014 valuation. Assume Uber had 600 employees at the time of the $3.5bn valuation.
The 2013 equity grants are now worth $700,000 to $2,800,000 (conservatively).
So I'd say 600-700 millionaires from that alone. Probably a whole bunch more if you count later larger grants times the smaller multiples.
So maybe 1000 millionaires?
Also, employees are only able to sell 50% of vested I believe in this round and will have to pay tax up to 40%. So that reduces the number a bit.