How do I know what IP to assign to my FreeBSD jail to avoid conflicts?

I set my jails up in a similar way, but I use a cloned loopback interface rather than an IP on an external interface. Since 127.0.0.0/8 (127.*.*.*) is reserved for the loopback address, you can use any address in that range (just don’t use the commonly used 127.0.0.1). I prefer to reserve 127.1.0.0/16 (127.1.*.*) for jails, so I could use the 3rd octet to logically/mentally group them if needed (e.g., by purpose or function). This approach “feels right” to me since it keeps local services on local IP addresses.

Add to /etc/rc.conf:

# Jails:
ezjail_enable="YES"
cloned_interfaces="${cloned_interfaces} lo1"
ifconfig_lo1="inet 127.1.0.0/16"

Bring up the cloned interface: sudo service netif cloneup

Add to /etc/pf.conf:

ext_if="vtnet0"
jail_if="lo1"
jail_net=$jail_if:network
www_jail="127.1.1.1"
www_ports="{ 80, 443 }"
nat on $ext_if from $jail_net to any -> ($ext_if)
rdr pass on $ext_if inet proto tcp to port $www_ports -> $www_jail

Create and start your jail:

sudo ezjail-admin create www 'lo1|127.1.1.1'
sudo ezjail-admin start www

The rest (such as installing ezjail) should be typical setup/config.