Hi Anil,

Sadly there isn't really an official documentation that gives you exact numbers. It is a bit of a calculated guess.

1. If you have 8GB of RAM with 4 cores it should operate just fine. Be sure to have enough storage available too!
2. The postgreSQL keeps working pretty nice and the amount of records doesn't have too much effect. Do mind that the PostgreSQL needs enough RAM to be able to process all data. A lot of things can be tuned in the database to improve performance. Have a look at this documentation: https://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server
3. Most people use Ubuntu 14.04 and it works excellent, haven't had any problems with Ubuntu 14.04 yet. You could also use CentOS but there is less documentation available for this. Just don't use Windows when you're rolling it out for a customer or so many users, the results are not as good on Windows.
4. Both Google Chrome and Firefox do a great job. In theory all browsers are supported by Odoo but I wouldn't suggest Internet Explorer or Edge, these have more issues than Google Chrome and Firefox.

Yenthe