I think but not positive there is 9 head servers on the magic and wonder and 12 on the dream and fantasy.
That’s also my recollection, based on observations on Magic, Wonder, and Fantasy. Each dining room accommodates approximately 1/6 of the total ship capacity (from 3 dining rooms with 2 dinner seatings nightly). Three head servers per dining room on Magic/Wonder and four per dining room on Dream/Fantasy.
On the Magic/Wonder, there are about 875 staterooms (I have seen 875 in some reports and 877 in some others). With an average of 3 passengers or so in each stateroom (just a guess for purposes of estimating and to simplify the calculation), each head server would have 1/9 of about 2625 = 291 diners.
On the Dream and Fantasy, there are 1250 stateroom, so about 3750 passengers. With 12 head servers, each would be responsible for about 313 diners.
So, my guesstimate would be that each head server has about 300 diners, more or less, say +/- 10%, depending on how full the ship happens to be.
If a serving team has 4+6+8 top tables, there would be about 8 or 9 serving teams per head server (18x8x2=288, 18x9x2=324, with two dinner seatings).