what subnet should i use?
i have 1 router, so just have 1 default gateway..
i need 500 usable host, because i have 500 clients..
and this router will connect to 18 access point...
can i have different network for every access point, each access point i want 30 useable host..

if the network too big or have too many host, it will slow down the speed it is? because every host need to wait to broadcast...

how to design this network? what subnet should i use?

/23

500 hosts is not that much, but it will still impact network performance, because they will not vait to broadcast, all the hosts will just broadcast generating considerate load on the router.

an dyes, you can use /23 network, that will double the host count of /24.

However i would recommend to consider possible expansion, and already create several smaller networks and route between them.

The type of router, method of connection the APs and typical traffic patterns (e.g. is there significant client<>client traffic) also have a bearing on optimal network design. More information would be helpful.