I have marked break_on_match as false, it goes through both pattern
But that's not what you want to do. You want it to try two patterns and be satisfied with the first match. The first expression could match if the log message looks like JSON and begins and ends with braces, i.e. your two expressions could look like something like this:
(?<LineNumber>[^a-z]+) - (?=\{")%{GREEDYDATA:json}(?<=\})$
(?<LineNumber>[^a-z]+) - %{GREEDYDATA:LoggedMessage}
The (?=...) and (?<=...) are zero-length lookahead and lookbehind assertions. Note the double quote in the expressions; make the whole string wrapped by single quotes instead of double quotes to avoid problems.