You can increase this value by running:
sysctl -w net.ipv4.netfilter.ip_conntrack_max=12000
This will increase the limit to 12000, however be aware that each tracked connection eats
about 350 bytes of non-swappable kernel memory!
To ensure this setting is restored on reboot add net.ipv4.netfilter.ip_conntrack_max=12000
into /etc/sysctl.conf. Obviously change 12000 to something which is suitable for yourself.
You can keep track of the current number of tracked connections by running wc -l
echo "8192" > /proc/sys/net/ipv4/ip_conntrack_max