Documentation of plural rules
These plural rules are managed and documented in Unicode CLDR data. You can check these rules there:
http://cldr.unicode.org/index/cldr-spec/plural-rules (for the documentation, with links to the actual data, comparison tables, and online tests)
See the chart here:
http://www.unicode.org/cldr/charts/latest/supplemental/language_plural_rules.html
I don't think we need to duplicate this international effort by a local page which may fall out of sync (and the top of the article already says that it is out of sync).
Most probably the wikitable should be removed from this incomplete draft page, and instead you can direct to the chart link above.
But note that this wikitable includes some plural rules for languages that are still not specified in the released CLDR data, such as Abkhaze (ab), which was not published officially because it was still not vetted but only drafted: That's where we could add a section below the reference URL above, showing draft rules currently used by MediaWiki for these missing languages.
But Amharic is included in released CLDR data.