https://www.atf.gov/explosives/qa/black-powder-subject-regulation-under-federal-explosives-laws.
No. "substitutes" are "propellants" not "explosives". As an aside; Smokeless or nitro powders for hand loading are also "propellants".
Honestly? The issue of "how much"? Is more closely regulated by an individuals local State, County and City laws and regulation.
Example; (Local for me);
1. I live in a City, they say I can have only 10 pounds (of BP) with no limit of "propellant"
2. A friend lives 1 mile down the road outside of the city, but same county,, he can have 20 pounds
3. Another friend lives 5 miles east, outside the City, different county,, he can have 50 pounds
Uhm(?) no.
What they all worry about is local "Fire Department issues". Ya know(?) Are ya in an old part of town with houses 15 feet apart? Or are ya in Walden woods with 15 acres between houses? Rural? got a neighbor a 1/2 mile down the road?
A simple call to the local FD Chief will answer all of your questions, the true answer is, it's a local issue for each of us.
p.s.
I suppose, so bob,, if you call the local FD? Use a burner and don't give'm your name!!