If I am understanding correctly, you have hundreds/thousands of files, each with a particular naming scheme. Inside each of these files, you have the word number at least once, and you want to replace that word with part of the filename (extracted from the filename, based on the scheme you mentioned).
Notepad++ search-and-replace does not have access to the filename, so it cannot be done natively inside Notepad++.
However, by a clever combination of command-line and Notepad++ manipulation, you may be able to implement something that will work for you. For example, earlier this year someone asked about just inserting the whole filename into their file(s) overtop of a PLACEHOLDER variable in the discussion here – my reply, and the follow-ons, show one way in which this can be accomplished. Using the same command-line portion, and then tweaking the regex to use number instead of PLACEHOLDER should get you pretty close to what you want.
Good luck.
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Useful References Please Read Before Posting Template for Search/Replace Questions Formatting Forum Posts FAQ: Where to find regular expressions (regex) documentation Notepad++ Online User Manual: Searching/Regex