The short answer
Do the arithmetic backwards from the number. Take next year's new ARR target, divide by realistic per rep yield, then divide again by the attainment you actually expect, which for most teams is around 70 percent of quota capacity, and you have your headcount. Then subtract the ones you already have, add back for attrition, and pull the start dates forward by the ramp period, so a rep who needs to produce in Q3 has to start in Q1. Order matters: AEs before SDRs, a sales engineer before your fourth AE if the demo is technical, and RevOps the moment you cannot answer a pipeline question in ten minutes.