NORTHAMPTON — As merchants brace themselves for crowds this tax-free weekend, some advise their customers to decide on their purchases the day before to avoid some of the hassle. “Try to picture 32 bodies” in this store, remarked Ken Adams, sales associate at Besko’s Appliance, a small home and kitchen appliance shop on Cottage Street in Easthampton. That was the number of customers he recalls were waiting for him at one point during last year’s sales tax holiday.