[ScottOliveira]

I don't believe there is any limit to the number of balls you can carry (other than the limit on empty cells).

There is definitely a possibility of a strategic advantage by hording balls. Every ball you possess can not be scored against you. However, if you aren't using the balls, and are simply holding them, then all of those balls can't be used to score FOR you either. The advantage would come down to the other robots on the field. If the other team has a bunch of very accurate scorers, and the robots on your team are not as good, it would be a good idea to prevent the team from getting those moon rocks.

However if your team has some excellent scorers then those balls aren't helping you any. If you have some way to pass the balls to your teammates however, they can help you, but bear in mind that it takes time to pass the balls. Is it quicker to pass than to have the other team collect them on their own?
If you can hold on to some and then give them to your teammates later on when orbit balls are likely to be scarce, it could provide a nice scoring boost at the end of the game and be a huge advantage.

Also consider how many balls you'll be able to get a hold of. Remember that at the beginning of the game you can only hold 7 in each robot, and the human players get the rest. So there are no free balls on the field. If lots of the balls get scored, you may never be able to reach 20 or whatever your maximum ends up being. There will be 5 other robots on the field, who may also all be trying to pick up more balls. Is it likely in a given match you would be able to pick up 13 more balls? Probably. Is it practical or the best use of your time? Maybe, maybe not.