To that extent; that much/so many — referring to the degree or amount just mentioned or assumed.
あれだけ努力したのに、結果が出なくて残念だ。
It's a pity that despite working that hard, there were no results.
あれだけ急いでいたのだから、忘れ物をしても仕方がない。
Given how rushed you were, it's understandable you forgot something.
あれだけの人数が来れば、会場は満杯になるだろう。
If that many people come, the venue will probably be full.