Does this standard dcc send command you say, have the -l switch (/help Sending and Receiving Files) in it? to tell it to limit itself to the dcc server (/help dcc server) section "Max. Sends" amount specifified?.
And even if it does, i have still seen od occasions when this has been exceeded, its almost as if the section below isnt always matched when a send is started with a remote command.
Max. Sends
This limits the number of simultaneous remotely initiated DCC Sends mIRC will handle.
Note: This applies to users who request a DCC Send remotely ie. via a script file or via the DCC Server, not if you initiate a DCC manually.
I have however noted that in the times i have done it, the max send cap seems to always be respected if a Dcc server session initiates it, (note this isnt scripted fservers where files queue up, but just a simple fserve NICK says <NICK>SEND FILENAME in a DCC server session & DCC SEND NICK FILENAME occurs)