Most OKR sets fail the same way: the objectives are activities and the key results are deliverables. These prompts push the session towards outcomes you could measure without asking anyone how it went.
If you cannot state today's number, it is not a key result. 'Launch the new onboarding' is a task; 'activation from signup to first board above 40%' is a result.
Two or three per team. A set of six objectives is a list of everything the team was going to do anyway.
Keep a recurring board for the check-in so the original wording and reasoning sit next to the current numbers.
Two or three objectives with two or three key results each. More than that and the set stops being a prioritisation and becomes a backlog.
It has a number you can read off a dashboard, it describes an outcome rather than a delivery, and you know today's value when you set it.