With that many websites I'm assuming you'll also be running your own server farm? So you'll need more than just programmers; you'll need network support and server admins, a DBA or two and misc. support for all of that. And probably an IT director to manage it all.
I would suggest you seek out a good IT administration consultant, give him a few months to dig into the specifics of your business and have him come up with staff recommendations. Or hire an IT director first and give him the first duty of hiring his entire department.