Grand March (czech): minimum number?

Can you narrow the list down to people who will likely show up? However, avoid having a venue smaller than the number of invitees -- in case they all want to attend.

You can watch some videos on youtube and count the number of people in dance. This could work even if the dance varies from what you hope to do. You might even be able to count part of the way until it looks like it would meet your expectations.

I would say you can do a perfectly good Grand March with about 20 couples. Honestly, I've seen it more often at weddings that too many people try to do it and there isn't enough space, people drop out before you're done, there are large gaps if you have shorter/slower people, etc.

Edited to add: Also keep in mind that the standard dance floor that a lot of venues in the US have are not generally big enough for a Grand March. If you had most of your guest list show, I would make sure you have room around the edge of the space when you do splits, ideally big enough that you could do the two couple split (four people in a row) around the whole venue room if need be.