Hi TECServices
Whilst Komal_S is technically right - there is no real way for you to ensure that each app isn't writing the same data - that is the nature of the API - however, that sort of thing (conflict) is (should be) catered for programatically.
And further - the chances of 2 apps writing to the same item at exactly the same time is almost impossible - however, if say an existing entity (invoice/purchase) is being edited by 2 entities - the integration software should check that between GET and PUT that nothing has changed on the GET information - and there are methods to do exactly that - and if it has changed - that the change now being PUT is still relevant.
But on the how many apps can access the API to a particular MYOB file - there is no limit - it is a gateway and each app just lines up and gets in the queue.
The Doc