The number of cores which will be available for recognition are defined in Processing Station Properties:
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In FineReader Server 14 you see physical cores of your processor. You can choose a number of cores and set N+1 number of processes. If your processor supports hyper-threading, you can set 2N+1 number of processes for optimal Processing station load (more information in help: https://help.abbyy.com/en-us/finereaderserver/14/help/idd_ocrstationproperties).

Each multi-page document is divided into portions (25 pages each by default). Each process receives one portion for processing (more information in Attributes > PagesSlice: https://help.abbyy.com/en-us/finereaderserver/14/help/configuration_file).

If you are processing a small number of documents at the same time, for example, two 10-page documents, only two cores will be used. For more cores to be used at the same time you can send a larger batch of documents or change the number of pages in one portion in Configuration.xml (help link above).