Let’s cut to the chase. Just because you can create a tab for every custom object you create, doesn’t mean you should.
The question then becomes, when should you and when shouldn’t you? Here are some questions to help you answer that question.
1. Will the object be referenced directly?
If it’s like an Opportunity Product, and/or associated with a master-detail lookup, it probably doesn’t need its own tab. This is because it may not be contextual enough without its parent.
2. Will the object be used often?
If your client will need to access this object often and quickly, then it makes sense for it to have its own tab. This is regardless of the answer to question 1.
3. Which application will use it the most?
While there are most likely multiple apps available from your app launcher, not all of them need to show the tab. Be selective with which app displays the tab.
4. Are you using a Lightning app?
You cannot always predict with 100% accuracy how users will actually use an app. So confirm all used apps are lightning-enabled. This allows users to personalize their own experience by adding tabs at their discretion.
The takeaway
Keep things super simple by only creating a tab for objects that actually need one.