Tufts Medical Center nurses and management met for 20 hours Sunday and Monday and were close to agreeing on a new contact that would avert a strike on Friday. But as often happens with serious labor-management disputes, the nurses won't decide whether to strike on Friday until the very last minute. Another negotiating session is not scheduled until tomorrow. Nurses said yesterday that one last issue stands in the way of a resolution: whether nurses on the day and evening shifts at the Boston teaching hospital can be required to care for six patients at a time, or whether they win language limiting the maximum number of patients to five.