Curious question about glasses

What glasses prescription is considered legally blind or is that how they determain it? I'm semi new to this. My 2 year old has glasses and refractive amblyopia. His prescription is a +7.50 they said his prescription was extremely high. My cousins glasses at a lower prescription and shes legally blind in one eye so I'm just curious if anyone can give me some advice or answers I probably sounds crazy lol but I'm so curious because I have no clue about glasses. I'm more educated now on his amblyopia but the far sightedness I have no clue!!