Thank you for the question.
May I suggest that you include the parentheses within the italics applied to lesson references (the print in your attachment seems to have them italicized)? Then it would seem cut and dried that you should use the three-cell Nemeth type-form indicators around all such references.
If you do not italicize the parentheses that enclose lesson references, it would still be best to use the three-cell Nemeth type-form indicators for a sequence of two or more lesson reference numbers.
Either way, please keep in mind the possibilities that: 1) lesson references may fit the Nemeth Code definition of "enclosed lists," and 2) your variable n may not require any italicization (i.e., if, as is very common, print makes no distinction between the meaning of italicized n as opposed to unitalicized n, then there is no need to italicize n anywhere).
Again, thank you for sharing your clear and focused question!