And thinking about making a short barrel jr  carbine. my question is can I get one stamp and do both 9 mm and 45  conversions

is it one  stamp for each barrel.

I have searched for a couple of days to find something similar No luck.

Not familiar with that particular model, but the stamp is "assigned" to the part that is considered the firearm, the frame or receiver.

If you could convert all the other parts between the to calibers and still use the same receiver, then it's just one stamp.

Same as, for example, having an AR15 SBR with two complete uppers in 5.56 and .300 blk, they use the same lower and are just one SBR with one stamp.

As long as you're not replacing the serialized part (which I believe is the upper portion of the receiver on the JR) with another SBR configuration, you will be just fine.
Form1 it as one or the other and then you can swap caliber back and forth at will.

The ATF "recommends" that you notify them of any permanent change in barrel length or caliber.