644 and 755 are the two most useful octal codes to remember because they make up the majority of files on your system. 644 is user read/write but read-only for everyone else. 755 adds execute to that, useful for scripts and directories.
Other than that, the most common other things are setting access for group and others to zero, so your ~/.ssh directory is 700 (rwx for you, no access for anyone else) and the private keys in it are 600, rw for you, no access for anyone else).
644 and 755 are the two most useful octal codes to remember because they make up the majority of files on your system. 644 is user read/write but read-only for everyone else. 755 adds execute to that, useful for scripts and directories.
Other than that, the most common other things are setting access for group and others to zero, so your ~/.ssh directory is 700 (rwx for you, no access for anyone else) and the private keys in it are 600, rw for you, no access for anyone else).