Hard to say, and I suck really bad at estimates. If I low-ball it at maybe 200-300, that's likely off by a factor or three. A possibly old but close enough page on chromium.org lists the following teams:

    Animations Team
    Binding Team
    Device Team
    DevTools
    DOM Team
    Ecosystem infra
    Input Team
    Layout Team
    Paint Team
    Rendering Core
    Speed Metrics Team
    Style Team
    Web Capabilities (Project Fugu)
    Worker Team

and that looks like it's only focused on Blink, not including the actual chrome (desktop/mobile apps) or V8 which would both be substantial efforts as well. Who knows how much technology is pulled from other teams at Google, like video encoding for WebRTC, server performance collaboration for the SPDY/QUIC effort, low-level storage for IndexedDB, devrel with buggy websites or certificate authorities, security audits and whatnot?

For doing the "best job possible", you can probably blow up team size by a lot, there are numerous tasks that would benefit from more attention and I'm sure Mozilla are already making hard trade-offs at their current staffing levels (which obviously include a lot of people outside of Firefox engineering).