We have multiple different apps, each deployed in multiple environments, each monitored by a separate Application Insight instance. For example 2 web applications, each deployed in dev, test, prod environments, that means 6 different Application Insight instances.
Microsoft wants to migrate Application Insights to workspace based Application Insights, so I need to create Log Analytic workspace(s). What is the best approach and why:
[I'm a part of Application Insights team]
Overall the recommendation is to keep the number of workspaces to a minimum unless you need clear separation:
This allows you to manage less number of resources (workspaces).
So, you should make a decision based on #2 - #5 (as mentioned above - auth is not relevant because it will still be controlled by Application Insights).
If you're not using advanced features (different retention), then most likely the main driver is different environment. I.e. in your case it is probably 3 workspaces (dev, test, prod).