Many of the people I know who did grad school went to a different school than their undergrad. So up to four, I'd think. Although, you could really get an AA, a BS, a MS, and a PhD from four separate schools, but getting into a community college doesn't require legacy admissions.

Does your parent need to graduate to be considered a legacy?

My dad went to 3 different undergraduate colleges each of his 3 years of undergrad, kicked the MCAT's teeth in, and got into med school without having graduated, went to two different med schools. (A long long time ago, probably not possible now.) Apparently the Mayo Clinic didn't mind his crazy academic record, and once he finished his residency at the Mayo, nobody else cared.

Mom went to one college, so maybe I would have been a legacy at 6 different institutions.

Legacy includes more than just parents.

Doesn’t always help though. My uncle was a star football player at Stanford who got drafted into the NFL. Wasn’t enough to get me in.