SAN FRANCISCO — The number of privately held Silicon Valley start-ups that are worth more than $1 billion shocks even the executives running those companies.

“I thought we were special,” said Phil Libin, chief executive of Evernote, an online consumer service for storing clippings, photos and bits of information as he counted his $1 billion-plus peers.

He started Evernote in 2008 on the eve of the recession and built it methodically. “A lot of us didn’t set out to have a big valuation, we’re just trying to build something that lasts,” Mr. Libin said. “There is no safe industry anymore, even here.”

But an unprecedented number of high technology start-ups, easily 25 and possibly exceeding 40, are valued at $1 billion or more. Many employees are quietly getting rich, or at least building a big cushion against a crash, as they sell shares to outside investors. Airbnb, Pinterest, SurveyMonkey and Spotify are among the better-known privately held companies that have reached $1 billion. But many more with less familiar names, including Box, Violin Memory and Zscaler, are selling services to other companies.