Basically Meraki's DHCP server will hand out IP addresses from the range that was defined for the subnet in the Security & SD-WAN > Addressing & VLANs page.
In your case the subnet is 192.168.0.0/23. I assume you know what that /23 means, but if you don't... It means the following:
-network address for the subnet is 192.168.0.0 (cannot be used)
-broadcast address for the subnet is 192.168.1.255 (cannot be used)
-host addresses go from 192.168.0.1 up to 192.168.1.254 (can be used)
-subnet mask is 255.255.254.0
In you configuration the MX is configured to use 192.168.0.1 as its own address, so that will not be handed out to clients, the rest of the host addresses will be handed out to clients: 192.168.0.2 up to 192.168.1.254.
The Reserved IP ranges function is there for IP addresses you want to keep available for static IP configuration, or to be used by another DHCP server or... So those addresses will not be handed out to clients by Meraki MX DHCP (unless fixed assignment, see below).
The Fixed IP assignments list is for when you want to assign specific IP addresses to specific MAC-addresses via DHCP. You can use addresses from the reserved ranges here, and it's probably a good idea to do so. This list also gets edited when you assign fixed IP-addres to a client from the Network-Wide > Clients page.
The screenshot you posted shows some details about this range. We see that indeed there are 510 usable addresses in total (192.168.0.1 up to 192.168.1.254), 1 is used by the MX, and 2 are used by other clients. This leaves 507 free addreses.