""" Under the GDPR, you must appoint a DPO if:
you are a public authority (except for courts acting in their judicial capacity); your core activities require large scale, regular and systematic monitoring of individuals (for example, online behaviour tracking); or your core activities consist of large scale processing of special categories of data or data relating to criminal convictions and offences. """
So if you're not doing 'large scale behaviour tracking', you would not need one. A simple company that sells a subscription service should not need one, unless they are also selling targeted ads, and maybe if they are doing identifiable tracking of how a given user uses the service. Aggregated metrics with no identifiable data do not count (This feature has been used X times). If you are, then it becomes a question of what is 'large scale' in terms of the GDPR.