Resolution:

In the Siebel adapter configuration in TIBCO Designer, under the 'Adapter Services' tab you will find two fields related to multithreading: 

Number of Threads to process Siebel Events (in the group 'All Publication and Request-Response Invocation Services') and Number of Threads (in the group 'All Subscription and Request-Response Services').

The first one is for configuring the number of threads to be used by Outbound Services (Publication and Request-Response Invocation services) at adapter instance level. The second one is the default number of threads to be used by each session containing Inbound Services. The configuration of the number of inbound threads can be done by the second field mentioned above as well as from the Multithreading tab, which is explained below:
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For inbound requests, the number of threads to be created during run-time has to be specified for each session (containing Subscription and Request-Response Services). A default value applicable to all these sessions can be provided through the field 'Number of Threads' in 'Adapter Services'
tab. This can be overridden for each individual session containing the inbound service(s), through the ' Multithreading' tab. Total number of inbound threads = Sum of thread count specified for each session (containing Subscription and Request-Response Services). 

So if the value of the parameter in 'Number of Threads' field in 'Adapter Services' tab is 1, and if there are five session containing inbound service(s), for whom the values are specified as <empty>, 2, 3, 4 and 5 respectively in the 'Multithreading' tab, the thread count of each of these sessions will be 1, 2, 3, 4 and 5 respectively. Please note that here for the first session since no value was specified through the 'Multithreading'
tab, the value was taken from the Adapter Services tab.

The best practice is to provide value for 'Number of Threads' in 'Adapter Services' tab as zero (0), and specify all inbound thread values through the 'Multithreading' tab, so that no unwanted inbound threads are created for any sessions.
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Siebel Adapter multithreading parameters for inbound and outbound scenarios.