iptables -t filter -I INPUT -p tcp --dport 25565 -j ACCEPT

iptables -t filter -I INPUT -p tcp --dport 25565 -m state \
  --state RELATED,ESTABLISHED -j ACCEPT

# Adjust "--connlimit-above NN" to limit the maximum connections per IP
#   that you need.
iptables -t filter -I INPUT -p tcp --syn --dport 25565 -m connlimit \
  --connlimit-above 10 --connlimit-mask 32 -j DROP

# Adjust "--connlimit-above NNN" to the maximum total connections you
#   want your web server to support
iptables -t filter -I INPUT -p tcp --syn --dport 25565 -m connlimit \
  --connlimit-above 150 -j DROP