Number of simultaneous FTP/FXP sessions and transfers

The following steps should work to update the setting, Max_per_ip in the vsftpd.conf file.
Note that each Isilon node runs its own independant copy of this software, therefore a change made here will not replicate to other nodes.  This would need to be changed globally on all nodes and then all nodes will need to restart the service for the change to take effect.
Please also note that these changes will not persist following an updgrade to the OneFS software.  So if this resolves the problem, it is recommended this change is logged in such a manner that it will be reviewed upon performing maintenance to upgrade the cluster software.

Step # 1: Open vsftpd.conf file

Use text editor such as vi to edit /etc/vsftpd.conf or /etc/vsftpd/vsftpd.conf file:
# vi vsftpd.conf

Step # 2: Setup limitation

Set up the max_per_ip=3 (this will limit simultaneous connection to 3 number only)
max_per_ip=3

You are restricting end user to a specific host to 3 concurrent connections. A setting of 0 should indicate unlimited connections per IP.  This change should be monitored as a large number of connections could have unforseen side effects on other processes. 

Step # 3: Restart vsftpd

# /etc/init.d/vsftpd restart

Now vsftpd should limit the number of simultaneous vsftpd connections for a single IP address.
If end user try to cross this limit she will see following error message:

421 There are too many connections from IP address.

Source: VSFTPD limit the number of simultaneous vsftpd connections for a single IP address

Thank you very much that helps a lot.

I looked on a few nodes at the config file but in no file there is the limit set.

Do you know what's the default (internal) value if no limit is set in the config?

VSFTPD has a default limit of 2000 connections overall and 50 connections per IP which sounds like it aligns with your previous post stating "round about 45 simultaneous ftp connections"