It would probably behoove you to contact Sales and/or your Sales SE as they might shed more light on capacity and features based on your environment. However, here's a previously posted link on Platform specs/matrix.
Even if not all 10.000 employees would browse at the exact same moment the thing to watch out for is the SSL-capabilities (which if you ask me is a must nowadays).
If you just look at the bandwidth (assuming 1Gbps uplink) a PA-3050 or even 3020 "should" be enough - but if you can afford a PA-5060 then why not :-)
Specially when the difference regarding concurrent SSL sessions are:
(SSL decrypt sessions)
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