adverb, noun which may take the genitive case particle 'no'
that much; to that extent; only that; that is all
Expresses degree, quantity, or limitation. Can mean 'to that extent' or 'only that much/no more than that'. Also used at the end of a statement to mean 'that's all I have to say'.
That's all you need to do, so don't worry.
That much is enough.
His success is the result of that much effort.
That's all. That's all I have to say.
He has ability to match that.
That alone isn't enough.
He's a hard worker; precisely because of that, this failure must be especially frustrating for him.
だけ is a bare particle meaning 'only/just' attached to any noun, while それだけ is the fixed phrase それ＋だけ meaning specifically 'only that much'.
Kanji form using 其れ (rare) and 丈 (ateji-like); rarely used in modern writing.
Mixed kanji-kana form; very rare and mostly historical.
Compound of それ (that) and だけ (only, extent). The derivation is transparent.