GACs are usually made out for 6 because most attractions have room for 6 in ride cars, waiting areas, etc. You will need to probably explain why you need a total of 8, so just be prepared (not because they won't give a GAC for more than 6, but because it is an exception to the regular).
As the prior poster mentioned, be prepared to split into groups of 6 or less if you are asked. When you show your child's GAC to the CM at the entrance, they will tell you whether or not you need to split. In some cases, they may just have the rest of your party go thru the regular or non-accessible part of the line.
Don't worry, the CMs will tell you what to do.
