First of all. Rough estimate is really hard to do, due to the volatility of the smash community. Being a TO myself, I know how annoying this is. However Smash tourneys almost always fluctuate in attendance, sometimes massively. And there are always last minute dropouts. Always.
However, in any case. Late November would be bad. REALLY bad. The weekend of the 19th, there is already a large Ohio tourney that a number of us are going to on Saturday; followed by a tourney in Grand Rapids the next day. Between the 2 of them, half our active players have already committed to at least one tourney that weekend, and a number of the rest have other plans that prevented them from committing to the first 2. The last weekend in November is Thanksgiving weekend, and a lot of people will either be traveling, or have family obligations.
I can't guarantee a ton of people for the 3rd, because there are a number of tourneys coming up. However you should be able to get in the double digits, especially if you have them advertise it to their customers/put fliers up. And that is enough to have a good time and get in experience.
But it's up to you what you want to do. Here's the tourney schedule for the MI scene as of now, to give you an idea of dates.
Nov 12th- Too soon to announce
Nov 19th- REVENGE
Nov 20th- Skynet
December 3rd- Melee tourney (no brawl tho)
December 10th- Phoenix
December 17th- DRDN's World/HOPE (is Hope on Sunday? not sure)
December 24th- Bad date for a tourney, obvious reasons.
December 31st- Another bad tourney date imo, people will have party plans/family plans/will be on vacation/home for winter break
January 6-8th- APEX