Here are few limitations of execute multiple request.
1. Cannot invoke execute multiple inside execute multiple.
2. Batch size limit is 1000. If you add more than 1000 it will throw a fault exception.
3. Only 2 concurrent executions can happen at a time, If you try running 3rd one, It will throw an exception saying “Server busy”
Defult batch size can be updated using UpdateAdvancedSettingsRequest. Here is the blog whichwill help you to update.
Hope it helps..!