I need to change look and feel of search result in a way that it looks same for all result types. Do I need to create a display template for each result type?
You do not need to create display template for each result type. Instead, you can create your own result type and one display template for this result type. When creating the result type for the sitecollection - do not specify any additional conditions. In this case it was used by default, and the results will be applied to your display template.