If the acceptance (qabul) is said wrongly or falters, how many times must it be repeated?

There is no set limit; the words are repeated until the offer and acceptance are complete and confirmed by the witnesses.
The contract is valid as soon as a correct acceptance, continuous with the offer, is spoken once. A slip with a name, a wrong dower amount or too long a pause may lead the witnesses or officiant to ask for a repeat, and that affects nothing. There is no need to feel pressured; the words need not be said in one breath, only not interrupted by something else that breaks the continuity. Practising the words before the ceremony helps a great deal. See Marriage contract.