This article did not shock me at all but somehow looking up the numbers on Docusign made me wonder, seriously, and with all due respect: how would you get to 7,400 employees at Docusign? I literally can't do back-of-the-napkin calculations that make it plausible.
https://news.ycombinator.com/item?id=33010050
1. Sales -> You can hire a lot of sales people if they're generating enough business to cover their own salaries
2. Customer Support -> Big enterprise contracts generally have support written into them. Thus, for every enterprise customer Docusign may need anywhere from 0.1 -> 3 (or more) FTEs. If you price this right it's always worth it to keep growing the org this way.
Docusign has a lot of opportunities to upsell customers on legal services they provide.
You should also consider the scaling cost of internal support. For every 10 FTEs, add a manager. Also add on HR, accounting, janitorial, etc. Easilly 20% of the company might be these sorts of internal support roles.