To answer your Kiwi Syslog questions:
1. 1) How many syslog license i should be looking at? - This depends on your volume. One syslog server can receive 2,000,000 messages when logging to one rule (display and log to file.
SolarWinds Knowledge Base :: How many messages can Kiwi Syslog Daemon handle?
2. 2) What kind of server configuration is required ? - By default none, the configuration is done on the remote devices.
3. 3) We need a log retention policy of 15 days. Should I consider to setup a DB to for log storage? - Kiwi Syslog can do log file rotation for you: Kiwi Syslog Server
Depending on the volume, you may want to log to a database.
4) Can the Orion inbuilt syslog write messages to external DB storage / Please explain this question more in detail.