Hi,
The 'engines at startup' is a legacy process mode cfg parameter, but it is used to decide what the initial
number of threads should be in syb_default_pool on threaded kernel (which is why Bret said to check it in the first instance).
As you found, after that you need to run alter thread pool for the syb_default_pool to adjust the threads.
This can be adjusted up to 'max online engines'.
The threads in the other thread pools fall outside of this parameter so you needn't be worried.
'max online engines' = max number of threads in syb_default_pool.
HTH,
Simon