In passive ftp in a single port how many connection can be made? or How many users can be connected at a time in a single port?
How many passive FTP connections can be made to IIS 7.0 on Windows server 2008 R2?
The manual states:
maxConnections: Optional unit [sic] attribute. [...] The default value is 4294967295 (unlimited.)
You won't be able to hit 4 billion connections though, the maximum amount of TCP connections depends on the operating system and available resources. You say you're using data ports 49152-65535, but you don't have to use a range that large - even if you have only one data port, multiple clients can connect to that port simultaneously, as well as they do to the control port (21), since the connection is identified using both parties' IP address and the source's source port and the destination's destination port.
But I must say: if you have to ask about the limits of some system, you're usually doing it wrong. Are there any problems you encounter with your current approach?