Hi,
1)What is the number of syslog messages that can be stored on my router with 8192 bytes
Log size is usually small (about 1k) So, you could potentially store about 8000 lines.
2)upon reboot of my router i notice the syslog messages would disappear. Is it possible to retain the syslog messages even when my router reboots?
No, it is not. It is always best practice to use a syslog server, so you can go back and look at historic data in case of a reboot.
HTH