none at all. Just don't leave it laying in the sun. Common the only thing that can really be "damaged" by pressure on a CO2 tank is the burst disk, and it only blows if the pressure spikes beyond whats safe....thus don't leave it in a car or in the direct sunlight for a long time.
Secondly...why? I have a sneaking suspicion this might turn into "how much HPA could I get into one". The answer to THAT question is none, unless you want to violate federal law. I'm not going to say anymore on the subject, and I suggest the moderators do not allow the discussion to continue in that direction for liability reasons. All CO2 pressurizes at the same OP so there is no need to know what the max is beyond illegally filling it with HPA. If the fool blows himself up, I hope the moderators and posters of this forum have the common sense not to have a hand in helping him..