Multi Customer application – Shared instance or per customer instance of application?

Hi I have an idea floating around my head to create a web application which might be usefull for some people, however there the need to store some sensitive data (to be honest is not that sensitive) and want to put the correct design inplace to prevent anyone for accidently seen any other customer data.

This has got me thinking when is the correct time to create multiple instaces of the application vs using a shared one. The multiple instances provide better security in terms of the only data in the DB and feels belong to that customer, however its very wasteful managing multiple instances and my applications does not need any heavy background jobs running.

What are peoples recommendations, at what point or what requirements would you develop your application to be multi instance for different customers.

Thanks