glorious-misunderstandings
"Twenty is a reasonable guess, if you're buying from a Pathfinder who can sell you one. You would use it inside a room with the door barred; outdoors is fine in a pinch, if you need shelter, but even keeping the space full won't stop anyone from shooting through the entrance or canceling the spell." There are better combinations of defensive spells to shelter under, but all of them involve passwords and can't really be rented out to the public.
"I don't know if you have gnolls on Earth, but most are not especially clever or proficient in subtle magic. Some can enhance their ability to track their quarry, but I predict with some confidence that they will not be able to follow your scent here. That is, if they bother to regroup and avenge their master, which I doubt, and if they know enough to come for you rather than me, which I also doubt." Without Ymohrglas to lead and outfit them, gnolls pose very little threat to anyone of Tanya's caliber. That's no reason to be complacent, but neither will Nazir suggest anything more extreme.
"The summoner would have the individual names of outsiders to call up, but there are three types that I think are well-suited to your question: burleevs, who scour Creation for magical lore, and who might be persuaded to part with their knowledge if you can find one interested in your magic; codex archons, who prefer to help those whose cause is righteous, but won't mind as long as it isn't harmful; and synesis aeons, who specialize in teaching dangerous and obscure spells. I would be surprised if you couldn't find one that knows the answer, since Abadar tells us there is an answer, but I will leave negotiating the deal up to you."