Can a commerce student pursue B.Sc Mathematics, if he/she had maths in 10+2?

Dear, Yes, commerce with maths can apply for Bachelor degree in maths honors because most of the universities require maths in class 12 whether you have commerce or science stream. But do check the eligibility of the particular university in which you want to apply. All the best.

Yes, one can, provided you secure the required marks to get admission.

Yes, you can pursue B.Sc after completing class 12th in Commerce.