expression, noun which may take the genitive case particle 'no'
to that extent; that much; that many
Refers to a degree, amount, or quantity previously mentioned or understood from context. Often used with a nuance of surprise or emphasis, similar to あれほど.
See also: あれほど
あれだけ練習したのに、試合に負けてしまった。
Even though I practiced that much, I lost the match.
あれだけの人数
I didn't expect that many people to gather.