My manuscript status says "18 reviewers invited." Is it a bad sign if that many reviewers were invited to review my manuscript? So far, that is the only information given to me by the journal.  I have no idea if all 18 reviewers were invited at the same time, or if one at a time.  In either cause, is this a bad sign, something like a looming possibility that the editor might eventually reject my manuscript if they can't find any reviewer to go thru my work?
It is difficult to give an informative answer without knowing the culture of the journal to which you submitted.  In the journals where I am an editorial board member, 18 would be a high number to get the number of reviews necessary for a decision (often 3).
A couple of reasons to be pessimistic:

It could be that your manuscript is being handled by an associate editor (AE) who likes to fire off a huge number of review invitations routinely, in which case I would suspect that this AE does not work hard to find good expertise matches.
It could be that the information from which the potential reviewer has to decided whether to accept the invitation (author list (unless double blind), title, abstract (usually)) is not enticing the reviewers to accept.  If this is true, you could try to take this as constructive feedback that your title and abstract should make someone want to read your manuscript.

On the optimistic side, I do not think that having a large number of reviewers invited is particularly bad news for the ultimate acceptance of your manuscript. Best of luck.