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
or honestly accurate thats why majority of people on here are the ones who dont actually own a casino but in terms of exposure and advertisement then majority do knows on what would be the best way considering this forum has the largest crypto traffic which its just really sensible that you would really be considering on running any campaigns or giveaways or something like that as long it could
hook up people on visiting the site and end up on a potential user.