Parents ask me this expecting a number, eight, ten, twelve. The honest answer is that the number matters far less than the mix.
I ask every family to build three categories. Reach schools, where admission is a real possibility but not likely. Match schools, where the student's profile genuinely fits what the university is looking for. And safety schools, where admission is highly likely and the student would still be genuinely happy attending.
The mistake I see most often is a list that's almost entirely reach schools, with one weak safety school added at the last minute, one the student doesn't actually want to attend. That's not a safety school. That's just a backup you'll resent.
A well-built list usually has two to three schools in each category. Fewer than that, and you're not managing risk. More than that, and you're spreading the same amount of essay-writing energy too thin across schools that don't all deserve it.