Dear @Agota_Meszaros
The conventions for clinical trials in the OMOP CDM can be found here: Home · OHDSI/ClinicalTrialsWGETL Wiki · GitHub
However, keep in mind that they were developed for version 5.3 and have not been updated for version 5.4. Although the majority of the conventions are applicable to version 5.4, some recommended concepts might become invalid or non-standard, so you need to check Athena for their substitutes.
Regarding your question about the trial number: yes, I believe it can be stored as an Observation with concept id 4163733, and the trial identifier can be entered into the value_as_string field.