The best way to advertise your new casino is to put in a lot of work I'm into promotion in form of promos and giveaways, or if you want to make more noise on the forum you can do a signature campaign also.
Yes, some people will promote even scam casinos, right? 
.OP was looking for input from casino owners as he said "How much bankroll you had when you opened your first casino" but obviously none of the casino owners will support a new competitor so don't expect any answers from casino owners and the suggestions posted by users are quite honest and good as I was reading some of them. You can do signature campaigns or art contests like roobet and a few others do every now and then.
You can go with forum advertising as well. You can learn more about it here - https://bitcointalk.org/index.php?topic=5398346.0