I am from commerce stream and I am going to complete my UG in BCA this year. Is maths stream must or not for software engineering?

You may not be able to pursue M.Tech, but you can pursue MCA. Maths stream is absolutely necessary in almost all forms of engineering.

You are going to get BCA degree than and you have done your class 12th from commerce, so for taking admission for engineering. You need to be from Science background. So, I think so you should try for MCA after completing your BCA.

Maths is needed in software areas to deal with complex structures and algorithms. It is not mandatory, but you should have a sound knowledge about it.