Hello
There are 2 IITs which offer B.Arch course:

1. IIT , Roorkee Uttarakhand

2. IIT Kharagpur, W.B.

There are 2 types of Entrance test to engineering and architecture/planning courses in NITs and IITs.

1. JEE - Mains- (Paper-2, for B.Arch only)- For students aspiring for B.Arch 5 yrs degree to NITs .12th students of any stream with maths can appear for this. These are:-

a. NIT Hamirpur

b. NIT Bhopal

c. NIT Kozhikode

d. NIT Jaipur

e. NIT patna

f. NIT Trichy &

g. VNIT Nagpur

2. If you clear JEE Mains (Paper-1 for B.Tech only) then only you can appear for JEE Advanced ( Which is for IIT). And after 1st level(i.e JEE Mains) you have to appear for JEE Advanced (For B.Tech only). If you score an AIR (all india rank) in this exam , then only you will be eligible for AAT (Architecture Aptitude test , for IITs).

So on whole you have to appear for at least exams -(JEE Mains-JEE -Advance & AAT) to get admitted to IIT B.Arch courses of Roorkee and Kharagpur

Goodluck I hope this helps you