Just to clarify, a K1 entrant can apply for an SSN anytime after arrival regardless of when the wedding takes place as long as the I94 is valid. Most recommend doing it with at least 2-3 weeks left on the I94 authorization. In my wife’s case, we got married 2 weeks after her arrival on a K1, and since we were so busy with the wedding prep, we went to the SSA about a week after the wedding (~3 weeks after her arrival) and we were fine except the SSN was issued using her name on her passport rather than the name she took from our marriage. She changed that after she got her GC in her new name.