Objectives are the foundation of OKRs, and they need to be supported by key results. The number of key results for each objective should be between one and three, but it can be extended to up to five. This helps ensure that all members of the company are in line with their objectives and that the objectives are measurable. For example, if your goal contains 8 to 9 key results, it can be confusing to see if all of these key results relate to the objective or if they can be divided into different objectives.
It's best to keep the number of key results to a maximum of five per objective. In each layer, there should be 5 additional objectives combined with 3 to 5 key results each that reflect the work that needs to be done. The key results must be correctly connected in a reasonable way so that the employee can easily remember them. However, with between 15 and 20 key results per given objective, it's unlikely that you'll go more than a couple of weeks without completing a key outcome.
John Doerr and the people at Google recommend that each team have a maximum of 5 objectives with 4 key results per objective. My client created the third key result to ensure that every team could feel connected to the higher-level OKR.