I am planning to transfer 12 bottles of regular beer to a 1 gallon bottle. I will also be adding some oak spirals to the 1 gallon bottle for aging of the beer. I estimate that it will take about 6-7 minutes to transfer this beer to the 1 gallon bottle. My question is how much carbonation tablets will I need to add to the 1 gallon bottle to compensate for the lost of Co2 which will happen during transfer. Also how much oak spirals will I need to add to the 1 gallon bottle to improve this beer?
The only way (short of a lab test) to have confidence in the remaining carbonation level is to let the beer warm up to room temperature so that most of the carbon dioxide leaves the liquid. Then you can calculate the priming sugar additions as per normal.
However, please ensure your "1 gallon bottle" vessel is pressure rated! 1 Gallon / 4 litres of beer is a lot to lose in an uncontrolled beer fountain.
It's not possible to say how many oak spirals to use, but I would start with one. There's some good information here - http://www.fermentarium.com/homebrewing/brewing-beer/add-oak-to-beer/ .
Note that it's absolutely possible to over-oak your beer too. All those tannins can become mouth-puckering.
Perhaps it might be best to put the beer onto oak in the "1 gallon bottle", but do not add any priming sugar. Leave the beer for 1 week, then taste. Repeat until the desired amount of oak flavour is obtained.
Once you're happy with the flavour of the beer, then re-bottle with priming sugar to carbonate.