Adverb (fukushi), Noun which may take the genitive case particle 'no'
to this extent; to this degree; this much; this little
Usually written using kana alone
同じ虹を、これだけたくさんの人が撮影したものを見たことがありません。I've never seen as many people as this photograph the same rainbow.
Also written 是丈【これだけ】