You could create a field in your tumors table occurrence as a Summary field to count the records (rows) in the related portal and use the running balance to number them according to your sorting directions. If you want to show the total count or calculated it to a field in your patient's record, then use another summary field to get the total count. This method will change the numbering and total count whenever you delete an existing tumor record.
Each new tumor record, and actually all table occurrences, should contain fields to automatically create the date when the record was created, a date when the record is last modified, modified by whom, and a serial number.
To add to that, "I also want "Tumor ID" to be generated accross all tumors of all patients." sounds like you want to profide a unique identifier for each tumor record. Define an auto-entered serial number field in your Tumor table for that purpose and use TECMan's technique for counting tumors for a given patient.
thank you! that worked beautifully. Yall are great!!!