Resulta que a pesar que los parámetros de Pureftp desde cpanel si cambian, pure tiene maximo 50 usuarios hardcoded. Levanté un ticket al proveedor del server del trabajo, y esto me respondió:
Alfonso,

Maximum number of ftp users was increased to 150 as per your request:

# ftp dominio.com
Connected to dominio.com.
220———- Welcome to Pure-FTPd [privsep] [TLS] ———-
220-You are user number 1 of 150 allowed.
220-Local time is now 20:50. Server port: 21.
220-This is a private system – No anonymous login
220-IPv6 connections are also welcome on this server.
220 You will be disconnected after 2 minutes of inactivity.
500 This security scheme is not implemented
500 This security scheme is not implemented
KERBEROS_V4 rejected as an authentication type
Name (dominio.com:root):
#

While going through pure-ftpd source code I found that it limits maximum number of users to either value of the MaxClientsNumber configuration option or number of available passive ports divided 2. Previously PassivePortRange was set to the «30000 30100» value so maximum number of clients restricted within the pure-ftpd source code was auto-set to (30100-30000)/2=50. I changed PassivePortRange to «30000 35000» and set MaxClientsNumber to 150 to achieve your needs. Hope this helps

Regards,
Paul