As a workaround to this limitation, I use two rules to cover all the cases.
For example, if I want to allow or deny these 18 ports:
465,110,995,587,143,11025,20,21,22,26,80,443,3000,10000,7080,8080,3000,5666
I use the below rules:
iptables -A INPUT -p tcp -i eth0 -m multiport --dports 465,110,995,587,143,11025,20,21,22,26,80,443 -j ACCEPT
iptables -A INPUT -p tcp -i eth0 -m multiport --dports 3000,10000,7080,8080,3000,5666 -j ACCEPT
The above rules should work for your scenario also. You can create another rule if you hit 15 ports limit on both first and second rule.