Which one makes you happy? -> the singular is correct She will have many children -> the auxiliary is "will", you can't use the third person after the auxiliary

After which, it depends. One will be singular for obvious reasons (there's a clue in the word), but we would say: "Which activities do you prefer?" if we are referring to something that is plural. The verb will is a modal verb. It should be followed by a form of the infinitive called the zero infinitive. You form the zero infinitive by leaving out the 'to,' for example "to run," should become simply "run." So we would say "I will run." "They will run." "79 elephants will run." In the same way, we would say "have," rather than "has," because you have to use the infinitive form minus the 'to,'