One tweed at least (especially useful in US winters, and not at all unusual in an academic setting). Colors and patterns are up to you, but you should probably stick to muted patterns until you have the basics down, or are very confident of your style.
If you have the option of going bespoke, some good books to look through are the Alsport (12oz primarily for your use, I think, but also 16oz), Porter & Harding Glorious Twelfth, any number of Harris tweed collections. Holland & Sherry Snowy River fabrics are not tweeds, but have many excellent jacket patterns.
Best weights for year-round (not summer, but other seasons), taking central heating into account: 11-12 oz, although you may find yourself needing 14-16oz and higher during the winter. For some reason, a really heavy tweed jacket feels less oppressive to wear in warm weather than a flannel suit.