sysctl -a shows settings for conntrack, amongst other things. This will show count of number of entries in table as well
ip_conntrack has a default timeout of 5 days (432000 seconds) for *established* connections. That is, the entry will be kept in the table for 5 days before expiring if there is no traffic (?). With a large amount of traffic, this could grow very large.
There also settings that control SYN, ACK etc for conntrack (check this and see below)
AFAIK, the only way to clear the current count is to unload the relevant modules (check this)
You can put customisations that override defaults in /etc/sysctl.conf, which are read when sysctl is run, usually at boot. Warning:
When the ip conntrack module(s) is/are (re)loaded, the defaults are used. You have to run sysctl to read any settings in /etc/sysctl.conf (sysctl -p /etc/sysctl.conf)
net.ipv4.ip_conntrack_max = 1000000
net.ipv4.netfilter.ip_conntrack_tcp_max_retrans = 3
net.ipv4.netfilter.ip_conntrack_tcp_be_liberal = 0
net.ipv4.netfilter.ip_conntrack_tcp_loose = 3
net.ipv4.netfilter.ip_conntrack_tcp_timeout_max_retrans = 3
net.ipv4.netfilter.ip_conntrack_log_invalid = 0
net.ipv4.netfilter.ip_conntrack_generic_timeout = 3
net.ipv4.netfilter.ip_conntrack_icmp_timeout = 3
net.ipv4.netfilter.ip_conntrack_udp_timeout_stream = 2
net.ipv4.netfilter.ip_conntrack_udp_timeout = 3
net.ipv4.netfilter.ip_conntrack_tcp_timeout_close = 2
net.ipv4.netfilter.ip_conntrack_tcp_timeout_time_wait = 2
net.ipv4.netfilter.ip_conntrack_tcp_timeout_last_ack = 2
net.ipv4.netfilter.ip_conntrack_tcp_timeout_close_wait = 2
net.ipv4.netfilter.ip_conntrack_tcp_timeout_fin_wait = 2
net.ipv4.netfilter.ip_conntrack_tcp_timeout_established = 432000
net.ipv4.netfilter.ip_conntrack_tcp_timeout_syn_recv = 3
net.ipv4.netfilter.ip_conntrack_tcp_timeout_syn_sent = 3
net.ipv4.netfilter.ip_conntrack_checksum = 1
net.ipv4.netfilter.ip_conntrack_buckets = 8192
net.ipv4.netfilter.ip_conntrack_count = 598236
net.ipv4.netfilter.ip_conntrack_max = 1000000
