NetSuite is planning to change the way concurrency is tracked for RESTlets. Going forward, concurrency will be tracked per account. The previous governance was per user. This change will impact both SuiteScript 1.0 and SuiteScript 2.0 RESTlets.
After this change, we recommend a maximum of 25 concurrent threads per account. If you exceed the recommended number of threads, we will allow it if we have capacity at the time of execution. If not, we will throw an error. You will have to make sure you implement your integration to factor in this variability of throughput.