I have multiple services running on Service Fabric. I would like to add Application Insight for logging. I'm just wondering whether I have to add an Application Insight resource for each microservice or only one is common for all. What is the best practice?
There is no such thing a the best practice for this. It really depends. Some considerations:
If you do decide to use a shared AI resource there are options like custom telemetry initializers to include custom data that further identify which ASF application or service is sending the data if it is not included by default.
See also Add Application Insight to a existing Azure Service Fabric cluster for more info about how to integrate AI.
Now, when it comes to bring data together you do have some additional options that may or may not need additional services or configuration. For example: