打 has two readings. dǎ = the everyday verb 'hit / do / play' covered above. dá = the noun 'dozen' (loan from English), used ONLY in
一打 (a dozen) and
半打 (half a dozen). Separately, in the soda transliteration
苏打 (and its compounds
苏打水,
小苏打),
打 also reads dá — a separate loanword fragment with no relation to either the 'hit' verb or the 'dozen' counter.