Oracle Corporation released database version 19c in 2019. 19c is considered a long term release from Oracle Corporation and its extended support date is till March 2026.
Oracle Corporation does not recommend you to upgrade to 18c if you are already on one of the Oracle 12c database releases. So, if you are on either Oracle 12c R1 or Oracle 12c R2, then the recommended upgrade is to upgrade to Oracle 19c directly.
Also, another point to note is that 19c is practically considered a patch set on top of the existing Oracle 12c database versions.
Oracle Corporation is now naming their database release numbers based on the calendar year similar to Microsoft SQL Server.
To give a background, Oracle 12c R1 was released in 2013, Oracle 12c R2 in 2017, Oracle 18c in 2018 and Oracle 19c in 2019.