How many km can be charged to my employer and is there any official publication on the rule? Also, if vehicle exp's exceed employer reimbursm't can I ded. remain'g exp's?

CRA does not allow the home-to office mileage as there is no deduction for "going to work" and "home to site" would be considered the same way.  All mileage from the office to anywhere else is considered to be valid travel.  There is the Canada Employment Credit that is supposed to help with the home to work cost. If you add up all of your fuel, interest, repairs, and insurance costs divided by the total mileage for the year times the allowable business mileage, you can compare that to the allowance you were given.  If your costs exceed that amount, you can claim the additional.  Be careful to keep all receipts as CRA is fond of asking for the support for this type of claim.

So if I started my trip from the site which is closeby home, is the milage from the site to the office chargeable, less the km from home to site?

You would have to run the finer points of that past CRA.  1-800-959-5525
The deal is that you drive from home to work, and they assume that once you check in at the office, you go to the site to perform your duties.  Making a pass through the job site to get to work does not satisfy the intent of the tax law.
Check the links here to see what you can claim based on your employment.  Once there you can use the search button to find out the limits.  You will also need a signed T2200 from your employer stating how much the allowance is and what you can claim.
http://www.cra-arc.gc.ca/employmentexpenses/

Is it possible to get the CRA reference stating that the home-to-office mileage is not reimbursable?

Here are two Canada Revenue Agency (CRA) that have this information:

Allowable motor vehicle expenses – Salaried employee expenses

Motor vehicle expenses