When Whatsapp was sold to Facebook in 2014, they had 55 employees. Considering the app had considerably less features and did not focus so heavily on encryption and privacy, Signal can be considered even leaner than Whatsapp.
Now, for the actual breakdown, they have at least the following technical teams: desktop, android, iOS, server, calls (ringrtc), core (libsignal). If we assume a team has usually 5 people (manager, Sr SWE, Jr SWE, QA, maybe PM), that’s already 30 people. On top of that, they have an in house support team (don’t know the size but I wouldn’t be surprised if they have 10ppl on the payroll considering the number of signal users) and management (CEO, CTO, CSO, VP), which will quickly add up to around 50.