You ideally need ~500GB of text, or so. EleutherAI's The Pile was designed to be just big enough to fit a 1t GPT efficiently, and you can get the various scaling curves out of the OA-related scaling papers. (You want the amount of data that fits into a single epoch, because if you reuse data, you get less bang for the FLOPs buck, and FLOPS constraints are right now much more binding than data or model size.)
Well, that's the "magic" of modern deep learning. You can fit models with p > n somehow without overfitting. In some areas you might find this called "the strong inductive bias of neural networks" or "double descent" but no one has found a convincing explanation (to me).
It's quite amusing. The standard statistical theory does not work at all in estimating data vs model size, and the bounds are all vacuously large. It's a very active area of research, understanding why models act so simple when overparameterized and coming up with real measures of model complexity. Lots to read there if you are interested in such things.