Hi Laura,
Most journal articles do have volume and issue numbers, so do look carefully in the article and the publication or site where you found it to try to locate them.
However, if they’re not listed anywhere, you can just omit either or both numbers. If you can only find an issue number but no volume number, it should still be listed in parentheses to identify it as an issue number, e.g. Journal Name, (4), …
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