Organization and bucket limits in OSS edition

In official docs Create an organization in InfluxDB | InfluxDB OSS 2.1 Documentation it is said that:
“A single InfluxDB 2.1 OSS instance supports approximately 20 buckets actively being written to or queried across all organizations depending on the use case. Any more than that can adversely affect performance.”
I analise potential usage of this tsdb for my project, but first I need know more about potential problems.
I’d like to understand what is meant by “actively being written”? Do you mean writing in the same write command or rather timestamp? If time: what frequency causes the problem?
Could you describe wider what is typical use case which ends up with affected performance?