How many Bs can I get in high school before I'm unlikely to get into an Ivy or Top 25 school?

As is often the case, it depends.

There are 35,000 US high schools but only ~15,000 Freshmen in the Ivy League. Less than 10% of applicants are accepted. With grade inflation and multi-way ties for first in class, there are 100,000–150,000 “valedictorians.” Even being a straight-A valedictorian, statistically, barely improves your odds.

Quite a lot of Ivy League/Top 25 schools admissions are influenced by factors in addition to grades. At some of the eight schools in the Ivy League, nearly half.

To answer your question, it’s necessary to determine whether you’re seeking admission based on grades alone or whether you will benefit from other factors.

If you are attending a typical public/private high school and seeking admission based on grades alone, even straight A’s are unlikely to be enough.

If you have some extraordinary extracurricular achievement, your odds improve. All-State athlete? Broadway role? Overcame homelessness?

Are you an Affirmative Action candidate (but not East Asian!)? This can help a lot.

Students who earn admission based on grades alone have taken the most demanding curriculum available to them and done extremely well. At rigorous prep schools, this usually entails 5–10 AP courses as well as honors-level courses wherever these are available.

Students from typical high schools might not have had the opportunity to take a lot of APs. Ivy League/Top 25 school applicants will benefit from taking the initiative to seek out more demanding curricula. This might involve self-study for APs or college level courses from local CCs.

Being a legacy can help to some extent. If, for example, you are a valedictorian at a typical public school and a parent graduated from Harvard, you will improve your odds above the statistical 10%. Maybe 20–30%.

Ivy League/Top 25 school admissions are extremely competitive. Setting this as a goal might lead to some disappointment. It will also position you well for admission to some other very competitive college.