Adding too much transmission fluid to your car or vehicles transmission can cause severe damage to the transmission and putting too much transmission fluid into your transmission and overfilling your transmission can cause transmission slippage, pump failure, and even cause your transmission to blow the seals.
When adding transmission fluid add a little at a time until it gets full.
A tiny bit overfilled won't hurt the transmission but if you overfill the transmission too much then you need to get it drained and refill the transmission fluid until it's at the correct level.