The instinct is to add another cheese when more people are coming. That is exactly where boards go wrong.
What grows is the piece, not the list
Double the guests and the instinct is to double the cheeses. But the rule that three to five is the useful range has nothing to do with headcount: the number of things one person can compare in a sitting is much the same whether two people are eating or twenty. Put eight cheeses out and they bury one another no matter how big the party is. What has to grow with the guest list is the size of each piece. Once that distinction is clear, the rest is arithmetic.
30 to 50 g a head, or 100 to 150 g if it is the meal
As an accompaniment, count 30 to 50 grams (1 to 2 oz) per person. If the cheese is standing in for the meal, count 100 to 150 grams (3½ to 5 oz). Aim at the bottom of the range when it follows a full dinner and at the top when people will be drinking for a while. Then multiply by the number of guests and divide by the number of cheeses. Six people grazing comes to about 240 grams, which across three cheeses is 80 grams each. That figure counts cheese only — bread and accompaniments are reckoned separately.
The numbers, by headcount
All as an accompaniment. Two people: 60 to 100 g in total, two cheeses. Four: 120 to 200 g across three, so 50 to 70 g each. Six: 180 to 300 g across three, so 60 to 100 g each. Ten: 300 to 500 g across four, so 80 to 125 g each. Twenty: somewhere between 600 g and a kilo across four or five — and from here it is better not to put it all on one board. If the cheese is the meal rather than the prelude, treble these.
For two, cut the list before the quantity
Two people grazing comes to 60 to 100 grams in total. Divide that between three cheeses and each piece is 20 to 30 grams — hard to buy that small, and cut that small the ratio of rind to paste tips badly one way. A soft cheese will not even hold its shape at that size. So for two, the thing to reduce is the number of cheeses, not the amount. One soft and one firm gives you all the contrast you need. If a third is really wanted, inviting one more person is the quicker fix.
A pair that contrasts cleanly when you are down to two:
Past ten people, split the board
More than ten around a single board and two things collapse. People crowd in, and the space you left between pieces for a hand to reach disappears. Then the temperature goes: it now takes well over an hour to get through, and soft cheese left out that long begins to weep fat. Making two or three smaller boards of the same composition and setting them in different parts of the room solves both. And when one board empties you bring out the next, which means the rest can wait in the fridge until then.
More of the accompaniments, not more kinds
Something sweet, something crisp, something sour. Those three do not multiply with the guest list — push past five accompaniments and the cheese vanishes under the garnish whether four people are eating or twenty. The quantities do have to scale, and bread and crackers go faster than anyone expects. Cheese left over with no bread to put it on is a dead end in the moment; bread left over gets eaten at breakfast. If one of the two has to be generous, make it the bread.
Better to have too much than too little
Aim at the top of the range. Running short is something you cannot fix at the table, whereas a surplus can be worked through over several days. What matters is which cheese is the surplus. Fresh cheeses want eating within a day or two, so leftovers of those become a chore, while hard cheeses keep for weeks. Put the margin on the firm side: parmigiano bought generously can simply be grated into the week, whereas burrata bought generously is best finished the day you buy it.
The ones that cause no trouble by being left over: