Hi There - Your question is quite interesting. OOTB BC is generally built to handle large number of transactions as it has got enough juice from Azure to be able to handle SaaS workloads. However there are no settings as such which will determine how many transactions can BC handle, so you can't really check/tweak anything before you go-live
However, if issues happen with OOTB BC, you can surely report to Microsoft
Also,
1. Do you have customizations deployed in your brand new BC Cloud Instance which will work post go-live?
2. Will your BC environment be integrated with any 3rd party website for fetching/sending data (3PL/4PL, etc.)?
The above two factors will largely determine how BC responds to large volume of transactions. The extensions deployed should be thoroughly load tested with proper QA and same should happen for your integrations. Loosely tested integrations/customizations can prove to be a headache. Make sure you deploy enough man-power now to test, than to worry later
Some other pointers to take into consideration:
1. Ensure there's no garbage data in masters (Often, there are blank rows of master data created, delete them)
2. Ensure you have your Job Queues scheduled at times when users aren't working (like the posting batch jobs)
3. Get rid of redundant data (If you have 3 customers named Google Inc, evaluate why you need 3 customers of the same name). Same goes for vendors. This will significantly improve your search results in SO/PO. Make sure to nomenclature your master data appropriately for search results to work faster
4. Deploy/Use telemetry in BC to be able to monitor workloads for a day/month to analyze where things go pretty bad and fix before London bridge collapses
learn.microsoft.com/.../telemetry-overview
