Since everything is federated, there will be multiple copies of the data across many servers so even if the original server goes down, the data is still saved on the other servers. Albeit without any images or any other non-text content which I think just get copied as links pointing back to the original server.
Now if you are just wondering about the sheer amount of text data, all of Wikipedia is about 20gb so while it is possible to run out of space, it probably won't happen any time soon. Images and video, on the other hand, are a different story.