I'd support dividing by number of authors, but it doesn't. A paper counts the same regardless of number of authors (and regardless or whether you're first author or not), unless it has "an atypically high number of authors for its field" (or something like that), in that case the committee can decide not to count it at all.

In Spain, gift authorship (adding authors that didn't do anything, typically in a mutual arrangement) is not uncommon. Not predominant, but definitely not uncommon.