I can't find limit information about Cloud Data Fusion.
Does anyone know, how many data pipelines can I create with Cloud Data Fusion by default? (link, source needed)
You can create as many pipelines as long as you are not hitting the quotas of the resources used in the pipeline. For example your pipeline uses BigQuery, Compute Engine, etc. and one of these hit a quota, then you are not able to create a new pipeline. See Data Fusion Quotas and limits for reference.