The guest list is a strategic choice. Who do you want to involve in this moment? Staff, former staff, clients, suppliers, partners, press, or a combination?
A smaller party (50–150) is more intimate and personal: you can give everyone individual attention. A larger party (200+) has more impact and reach. But such a party requires more direction to preserve the sense of connection.
Also consider separate moments: an exclusive dinner for the core group, and a larger party for the wider network. That way you combine depth with reach. That's the approach we regularly take for larger anniversaries.