Restrict parallel connections Using Iptables
Allows you to restrict the number of parallel connections to a server per client IP address (or client address block).
[!] --connlimit-above n Match if the number of existing connections is (not) above n.
--connlimit-mask prefix_length Group hosts using the prefix length. For IPv4, this must be a number between (including) 0 and 32. For IPv6, between 0 and 128.
Allow 2 telnet connections per client host
You can also match the other way around:
Limit the number of parallel HTTP requests to 16 per class C sized network (24 bit netmask)
Limit the number of parallel HTTP requests to 16 for the link local network (ipv6)