Well if # can be both part of your pattern and an indication that it is a comment to be removed, then you need to specify how to distinguish between the two cases.

With an input string such as:

pattern = the number is #8  # number
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this would remove everything after (and including) the last # of your string:

s/#[^#]+$//;
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but this assumes that you always have a trailing comment in your input.

But we have not way to know whether it will work with your other input lines (i.e. if there is always a trailing comment in your lines).