Information on the setting MessageChain.NumChains on the Advanced Server details page.
The number of messages, in parallel, that the File Reader will process. Setting this number higher than 8 (with the other default settings) is not recommended. A higher setting does not substantially increase performance and there is a much greater risk of running out of memory. Setting this to less than 8 (in some cases 1) helps when processing big files, but it may slow down the system considerably.
The default setting is 8.