Hi I am going to bottle my brew tomorrow which has been left fermenting for 6 months. It is as flat as water will I need to put more priming sugar in the bottles for it to carbonat?
If you were to bottle it as is, the beer will remain just as carbonated (if not less, due to agitation) than it is now. In order to get the carbonation you will need to add some source of fermentables (sugars) be it priming sugar or otherwise.
The type and amount of sugar added are largely dependent on style and personal taste. Though there are many an online calculator out there to help you decide that. Northern Brewer has both a useful calculator where you can put in your information and get a weight for sugar as well as a helpful and more detailed guide, should you care to read it.
While many will simply bottle condition by adding sugar to the beer and filling bottles, you may have a special issue. Having left the beer for 6 months, your yeast are probably dead. If they aren't dead they certainly aren't happy or healthy; this will lead to poor / slow conditioning. Think of putting a buffet in front of someone who has been starving for months, they'll want to gorge, but in the process will do themselves harm.
Ideally, you would create a yeast culture and pitch it into your beer once it reaches similar conditions of pH and ABV as your beer. More practically, I would recommend mixing fresh yeast when you mix in your priming sugar. This will ensure you've healthy yeast to carbonate your beer. This will have the added benefit of helping clean up any off-flavours that may have developed from prolonged maturation atop potentially autolysed yeast.
Nope. Unless you killed all the yeast with heat or chemicals.
Calculate the required amount of priming sugars needed for the carbonation that you need and you are ready to go.
I am going to guess that the beer will be slower to carbonate as most of the yeast would have fallen out, so maybe give your beer a week or two extra before chilling.
If the beers are not carbonated after a month, then you would have to add active yeast. Rehydrate 1 g dry yeast in one liter water. When the yeast is ready, add 1 ml per bottle and recap the bottle.