The default value is NumberOfThreads.DEFAULT, which means the search is performed using as many threads as there are processor cores (Environment.ProcessorCount).
The value can be changed at any time. A search uses the value that is set when the search starts.
A value changed while an index uses these settings is saved in the index immediately, or, if the index is performing an operation at that moment, when the operation finishes. A new index uses the settings with which it is created; an existing opened index uses the settings saved in it, which Index.index_settings returns.