How high a % of glicolic acid cream is safe to use just at home?

Glycolic Acid. This answer is not a simple one. Many of the over the 'acids' are partially neutralized so the percentage that they are claiming on the package is not the actual percentage. Another factor to consider is any underlying skin conditions such as rosacea or excema that can be flared up by the active ingredients. I would recommend doing a 'patch test' on a small area of skin of the neck.
Start Low. 5-15% glycolic acid facial creams are available without a prescription. Any acid may burn sensitive skin types. Always start out with the lowest dose you can find and then gradually increase if there is no irritation, every few weeks. Avoid combination acid probucts because you won’t know which ingredient caused any reaction.