Tom,
As you know the swap system adds in 1 extra dib for two packs "sold" on the first night, 3 dibs per the second night and so on.
A starting dib number of 3,4 or 5 makes it so that those who add the more desirable seeds are rewarded with extra tickets for their offerings while there's still a nice selection up for grabs.
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If that doesn't matter to you, you could look at the hostess dashboard. This will tell you how many packets are being offered in the swap and would help determine what the dib number should be set at to "sell" all listed seeds.
Another option is to change the multiplier mid swap if you see there's still lots to go around. I did this in the Mile High swap on the 3rd day. There were just so many offerings because there was a 100 pack minimum to join. I believe I had around 450 varieties offered. I "sold" almost all of them by changing the multiplier mid swap.
I'm trying to get a decent amount of offerings packed and entered. If I can't, I will likely drop from this swap.