If your Sales Executive has 300 Customer list, Manager should do individual Field Work and Follow up of Minimum 50 Core Customers in a month Your View.

That ll greatly vary industry to industry and manager to manager. 

If he is working individual behind50 core customers the sales manager is not playing the role of a manager but an executive.It is important that manager should try and get50 core customers regular in touch by means of emails ,messaging any activities engagement program with them .Field visits he can very well do with the sales executive during joint sales calls.The manager role in this position is to guide the field force and extract more business with motivation and regular rewards to the sales team and keeping achievable targets.