What You Can Decide Before the Replies Are In
Most of a seating plan does not depend on the final numbers. What you can settle while you are still waiting, and the two things that genuinely have to wait.
Most couples wait for the replies before they start the seating plan, on the reasonable-sounding grounds that you cannot seat people until you know who is coming. Then the last reply lands three weeks before the wedding and the whole thing has to be done at once, in the same fortnight as the final numbers, the meal choices and the running order.
Most of the plan does not actually depend on who replies. Here is what you can settle now.
The room, and how many tables fit in it
This is fixed the day you book the venue and it does not move. Ask for the floor plan, or the dimensions and what is in the room: where the doors are, where the bar goes, where the dancefloor and the top table sit, which pillars you have to plan around.
Then work out how many tables the room holds and what size they are. That is the shape you are seating into, and it is true whether a hundred people come or a hundred and twenty. If the room only holds twelve tables of eight, you know your ceiling before a single reply arrives. How to read a venue floor plan covers what to ask for and what the drawing does not tell you.
The households
A guest list is not a list of people, it is a list of households: the couple who arrive together, the family of four, the three friends sharing a taxi. Households almost always sit together, and grouping them is work you can do the moment you have the list, before anyone has replied.
It is also the work that makes everything after it faster. Seating forty households is a smaller problem than seating a hundred and thirty guests, and the households are what you will actually be moving around the room.
The keep-apart rules
Write down the pairings now, while you are calm about them. The divorced parents. The ex nobody mentions. The two cousins who fell out over the will.
Doing this early matters more than it sounds. Late in the process you are tired and looking at a nearly-finished plan, and the temptation is to leave a pairing you half-remember rather than rebuild two tables around it. Written down at the start, it is a constraint the plan is built with rather than one it has to survive. Wedding seating plan etiquette covers the common ones, and seating divorced or separated parents covers the hardest.
The top table
Who sits with you is a decision about your families, not about the response rate. It is also the decision most likely to need a conversation, and conversations take time you will not have in the last fortnight.
Settle it now, including the awkward version: whose partner is included, what happens if one side has more people than the other, and whether you are having a top table at all. Wedding top table ideas sets out the orders that work and the ones that cause trouble.
What genuinely has to wait
Two things, and only two.
The final numbers. You cannot lay tables for people you have not counted, and the caterer needs a figure that is right. This is the last thing to settle and it is the reason the fortnight before is busy.
The short table. Almost every plan ends up with one table that runs short, and which one it is depends on who declines. You can leave a gap for it in the layout, but you cannot say whose table it will be until the replies are in. Where to put the short table covers what to do with it when you know.
What this buys you
When the last reply lands, you are not starting. You have a room, a set of households, a list of rules and a top table, and what is left is fitting the people you now know about into a shape that already exists. That is an evening rather than a fortnight.
The Table Plan holds all four of those from the day you start: the room, the households, the keep-apart rules and the top table, so the replies land into a plan rather than in front of one. Start your plan, free for A$279 once, no subscription.