Max Number of Cc's when Adding More Saline to Implants?

Usually there is a a small amount of overfill that is allowed by the manufacturers, about 30cc in most cases. This makes little difference if the breasts are very different in appearance.
Exceeding this amount can result in deflation or rupture of the implant, or in a deformation of the appearance of the implant since it is made for a specific volume, and not to be excessively overfilled.
Dr. Canales has pointed out that implant deflation and weight changes can change breast appearance, as can capsular contracture, drooping or loss of breast tissue over time.
If you want to attain the very best result, you may want to have your situation reevaluated again by an experienced plastic surgeon rather than to try to add more saline to an 8-year old implant.

It is not a good idea to add volume to existing implants. The best option would be to exchange your current implants for new implants. You may also want to consider changing your saline implants to silicone implants. In either case, make sure you have a good discussion with your surgeon before embarking on yet another breast surgery. Good luck.

Breast implants have a recommended fill volume and these are relatively limited.
Although you can fill the implant beyond it's recommended range, it will distort the implant and likely produce an unwanted or distorted appearance, This may cause peripheral scalloping and an overly round appearance.
There are many reasons why the breasts may be different in size and these include implant leakage on the smaller side or breast gland growth on the larger side. These would require two different approaches to achieve the best result. Furthermore at 8 years, you may want to consider changing the implants.
Discuss these with a surgeon who will best be able to provide you with the optiuons for management.