How much overlap is permitted between a submitted journal article and a thesis?
Generally, many publishers and fields have rules against reuse of one's own text if that use is a (a) substantial; (b) not disclosed/attributed; (c) published.

How these terms are interpreted varies among publishers and disciplines. Most publishers would not consider a thesis or dissertation to be a "publication" for this purpose (hence the still widespread practice of converting dissertations into "book"). However, to avoid misunderstandings and keep definitively within the rules it is good practice to cite one's thesis, and to provide a brief summary of what content was reused and the extent it was updated, in the cover letter and in an introductory footnote.