For Allahabad, Lucknow and BHU universities, is 11th syllabus is asked in BSc Maths Group entrance exams. Or they only ask of 12th?
Nope! The syllabus will comprise of 11 and 12. If you have prepared for JEE or any entrance then it will be enough for you. In general BHU got students who are mainly JEE Aspirants not getting good branches in NIT's/ IIT's. So you will have a moderate competition with them.
To know more about the BHU UET exam you may visit the link provided below:
https://university.careers360.com/articles/bhu-uet-exam-pattern
I hope my answer helps. All the very best for your future endeavors!