<tt>src-nodes</tt> defines how many src-track (nodes) entries are stored in memory. By default 10000. This value should not be changed if you do not hit this limitation.
Source tracking is stored per-rule. Thus in the load-balancer each <tt>redirect</tt> has it's own source tracking. So in order to load-balance FTP both <tt>listen on</tt> has to be added to the same <tt>redirect</tt>.