If a file is coming from ftp and posting into SAP.
How many number of messages will be created , my doubt is whether it is number of records or per file one message?
Depends on what you are trying to do with data and what do u mean by the term number of messages.
Suppose the file contains data for multiple Idoc and you are sending IDoc by package, then there will be one message but internally multiple IDocs will get posted.
If you are doing one to N mapping and using BPM then one Idoc will go at a time and you can see multiple messages in SXMB_MONI.