I am not familiar how you can get that information. The closest one I know is the performance indicators which you can find in PROMON. Goto the R&D menu (which AFAIK is a hidden option on the main menu in Progress V9) - just key in R&D. From there go to option 3 (other displays) and then option 1 performance indicators.
Plus, I don't know if you would know how many transactions per second your database is handling provides any information of value. As one transaction is not equal to another how would you measure them against each other? The scope of a transaction and thus how large it is and how long it is running is totally determined by the application code. The much more interesting information IMHO would be whether you have long running transaction and what code (business case) is causing (rectifying) such long running transactions.
Heavy Regards, RealHeavyDude.