how to determine subnet mask?

A netmask of 255.255.240.0 will do the trick.

Your broadcast domain will be HUGE and you could have network performance problems.  If you need that many hosts, you would be better off subnetting

Both comments above will do the trick, I just hope for the netmask with that many host you have a manage switch and your running STP

i dont really need that much i guess. i was thinking of that many just so it wouldnt have to be reconfigured again.
it is for a guest wifi setup at a location that 1 time a year has a large conference of 2000 people.
this year about 700 connected to the wifi, they dont all use it at the same time, but they all had an IP address and i am using 255.255.252.0 right now so i was close to running out of available IP addresses. so maybe i should have phrased the question different? what would you suggest i do so the DHCP doesnt run out of IP addresses? thanks

If you do not want to manage one large blocks of ip addresses, you can have three or four dhcp running with there own scope. You just need a router configured properly to route traffic.

i would rather just have 1 block, no router.
what would 1500 ip addresses look like?
subnet and ip range?

Don't worry about it if it's for wifi - make the subnet as large as you need.  Wireless broadcast traffic is handled differently to wired.

For 1500 IPs you'd need a /21.  That's 255.255.248.0.  You'd get 2046 addresses in that range.

This might help you...

http://www.unixwiz.net/techtips/netmask-ref.html

great thanks.
would i use 192.168.0.1 to 192.168.8.254?

sounds like a good solution, thanks.
i dont see how to exclude from my sonicwall firewall (dhcp server)
but i will look into that.
i guess i could have multiple scopes that range from .1 to .254...but exclusion like on microsoft server would be better. cause then i could excude the gateway and the access point fixed IPs.

thanks