It totally depends for me on how long the ride is and what it is (supported/non-supported, remote, etc.).

Every ride: saddle bag with tools, 2 tubes, CO2, phone, money. Food and extra clothes if needed in jersey pockets.

For most of my rides as long as a double century, supported or not, I really don't carry much more than that.

Brevets over 300k: the whole shebang. Handle bar bar or trunk bag. I now have a rack and trunk bag with small panniers, but before I used a handlebar bag. Almost all our brevets that long are in pretty remote areas and they're all unsupported. 3 - 4 tubes and a pump as well as CO2. For a 600k, I bring an extra pair of shorts and long rain pants and a jacket, even if its not raining, for warmth. Back up lights and bulbs.

It also depends on the company: I just did a 30 miler with my daughter (her longest ride in her new bike with clipless) and I brought my brevet bike with a trunk bag with extra clothes, food, etc. We didn't end up needing any of it, but it was good to have in case and I really didn't care about the weight.