We focus on building a solid product with a consistent experience for several platforms - Wunderlist 2 will launch with native clients on Web, iOS, Android, Mac and Windows. To build a high quality product on every platform, I think you need at least 2 developers for each platform. If you are interested this is how we split our team:
3 iOS Developers
3 Android Developers
2 Mac Developers
2 Windows Developers
5 Web Developers (they not only build the web app)
3 iOS Developers
3 Android Developers
2 Mac Developers
2 Windows Developers
5 Web Developers (they not only build the web app)
5 Backend Developers
1 Sysadmin