It is a common practice to limit the number of Objectives and Key Results for the top-line and team OKR.
For example, one company set a maximum of 4 objectives, each with a limit of 4 respective key results. Thus, the maximum total number of KRs is 16.
The goal of this is to create a forcing mechanism to prioritize.
Two (2) influencing factors
A couple of factors that could influence your limit number:
- Team size
A longer time period (e.g. a year, instead of a quarter) might justify you to have a small — not many — additional KRs. The same applies to a larger team size.