A symbol used before can certainly be reused, in particular when it's meant to represent something very generic. For example, a length n on page 15 and a length n on page 21 may both refer to an arbitrary length, without any implied or otherwise statement that both lengths are the same.

The issue in your case is: Your supervisor doesn't like this.

There are, however, ways around this. A common way is to distinguish your symbols by subscript texts:

• For generic symbols, you can use running indices, e.g. n₁, n₂, etc.
• Alternatively, as that meight mean quite high (and arbitrary-seeming) numbers in later pages (x₃₄ = y₁₉₈ * z₁₂ might look a bit weird), you can add something specific to the chapter, the formula, or the figure (whatever the maximum scope of the symbols is supposed to be), and just a running number within that context. This would mean that symbols based on n in chapter 3.4 are named something like n_3.4,1, n_3.4,2, etc.
• As soon as your symbols are not meant to be entirely generic, but have a certain particular meaning or significance, a good way to name them is to use explicit indices that state the purpose of the symbol. So, you start working with n_{MethodX-input}, n_{JohnDoesCoefficient} and n_{result(algorithm1)}.

Depending on future plans for your text, you may even want to consider preferring such subscripts over any Greek characters or formatting variations (cursive, ...) in the first place, as the subscript variant can be flawlessly transformed/represented in contexts where little or no formatting is available (plain text files, unformatted e-mails, instant messages, hand-written paper or boardwriting, ...), and possibly (depending lastly on how you name your symbols) even with only ASCII characters that can be processed and recognized almost everywhere.

A symbol used before can certainly be reused, in particular when it's meant to represent something very generic. For example, a length n on page 15 and a length n on page 21 may both refer to an arbitrary length, without any implied or otherwise statement that both lengths are the same.

The issue in your case is: Your supervisor doesn't like this.

There are, however, ways around this. A common way is to distinguish your symbols by subscript texts:

• For generic symbols, you can use running indices, e.g. n₁, n₂, etc.
• Alternatively, as that might mean quite high (and arbitrary-seeming) numbers in later pages (x₃₄ = y₁₉₈ * z₁₂ might look a bit weird), you can add something specific to the chapter, the formula, or the figure (whatever the maximum scope of the symbols is supposed to be), and just a running number within that context. This would mean that symbols based on n in chapter 3.4 are named something like n_3.4,1, n_3.4,2, etc.
• As soon as your symbols are not meant to be entirely generic, but have a certain particular meaning or significance, a good way to name them is to use explicit indices that state the purpose of the symbol. So, you start working with n_{MethodX-input}, n_{JohnDoesCoefficient} and n_{result(algorithm1)}.

Depending on future plans for your text, you may even want to consider preferring such subscripts over any Greek characters or formatting variations (cursive, ...) in the first place, as the subscript variant can be flawlessly transformed/represented in contexts where little or no formatting is available (plain text files, unformatted e-mails, instant messages, hand-written paper or boardwriting, ...), and possibly (depending lastly on how you name your symbols) even with only ASCII characters that can be processed and recognized almost everywhere.