The answer is… it depends.
Assuming an even interarrival time (transactions arrive evenly throughout the day), then IS/TN would need to process roughly 4,167 transactions per hour or 69.44 transactions per second.
According to the WM Technical Note “Benchmark report for webMethods Integration Server 6.5 on an HP Integrity server running HP-UX” available on Advantage, IS 6.5 running on a 4-way HP Integrity rx4640 server running HP-UX 11i v2 can process 5,482 transactions per second when running a simple No-Op Java service and 51 transactions per second when parsing a 100K XML file. Throughput drops to just 4 transactions per second when parsing a 1MB XML file.
These benchmark tests were conducted for IS 6.5 and did not use TN. The bottom line, is that your performance will depend on many, many factors mostly including number of documents per unit of time, size of the documents and the work that must be performed on each. Of course, CPU, memory, network and disk I/O performance also impact the overall throughput.
The HP benchmark report is available on Advantage in the Best Practices -> Product Technical Notes section.