Multiple insance of same transaction (asynchronous mode)

Hello

I created a structured service composed by:

- a master transaction executed every 10 minutes which runs several instances (30) of a slave transaction with Asynchronous mode

- a slave transaction with duration between 1 minute and 3 minutes.

I found that

- master transaction complete its task in 10 seconds and runs all 30 instances of slave transactions (asynchronous)

- 18 slave transactions start at once

- 12 slave transactions remain in pendig status till first group finishes

Have you ever found this behaviour?

Is this 18 a fixed limit or a MII parameter? Where could I change it?

Thanks a lot

Regards

Fabio

ps: MII 12.2, netweaver 7.3