Who sits on the top table at a wedding
The top table is the row everybody has an opinion about, usually a parent. It helps to know what the traditional order actually is, because once you can see it written down you can decide how much of it you want to keep.
The traditional order
Facing the room, the long top table reads from one end to the other like this: chief bridesmaid, the groom's father, the bride's mother, the groom, the bride, the bride's father, the groom's mother, the best man. Debrett's sets it out in that order (Debrett's), and Bridebook gives the same line-up left to right while adding that there are no hard and fast rules about it (Bridebook).
Two things to notice. The couple sit in the middle, and the parents alternate so each of them is next to one of the other side's. Partners are deliberately split up, which looks odd on paper and works well on the night, because nobody spends the wedding breakfast talking only to the person they came with.
Why that order exists
Debrett's traces the layout back to a time when the bride's parents hosted and paid for the day, which is why the bride's mother and father flank the couple. Debrett's also says plainly that it is perfectly acceptable to dispense with these conventions and devise your own seating arrangements, and notes that the same principles adapt to same-sex partnerships. So treat the traditional row as a starting point rather than a rule you have broken.
Who else can sit there
Grandparents, a stepmum or stepdad, the person who raised you, a sibling who is not in the wedding party, or nobody but the two of you. Some couples seat the chief bridesmaid and best man at the nearest table instead, which frees two chairs on the top table for parents and step-parents. Bridebook's line is the useful one: you can ask anyone you would like to sit at the top table with you (Bridebook).
If somebody who matters cannot be on the top table, give them the next best seat in the room instead. A table directly in front of you, facing in, is more of a compliment than the far end of a long row.
Where the speech-makers go
Keep whoever is speaking near the middle and easy to reach. Debrett's advice is to keep the speech-makers, usually the father of the bride, the groom and the best man, central and at the same or adjacent tables. In practice that means checking two things with your co-ordinator: where the microphone will be, and whether anyone speaking has to walk past twelve chairs to get to it.
A sweetheart table or a round table instead
A long top table is not the only option. A caterer's guide puts the alternatives neatly: a round table for the whole wedding party, or a sweetheart table for the couple alone, and for blended families, swapping the single top table for two round tables (Dine). The same guide makes the case for round tables generally, since they let people talk to one another easily, while long tables seat more people in the same space.
A sweetheart table means you get ten minutes to eat together and talk to each other, which many couples say is the only quiet moment of the day. It also means the parents host their own tables, which can be the kindest answer when there are step-parents to seat. If that is your situation, there is more on it in where to seat divorced or separated parents, and the alternatives are set out in wedding seating without a top table.
How many a top table holds
Work it out before you promise anyone a seat. A long table takes as many chairs as the room allows and your venue will tell you its limit; a sweetheart table is two. In Sat Pretty, a top table seats up to 20 and a sweetheart table seats 2, which covers everything from the two of you to both families and the wedding party in one row. Ask your venue how wide the room is at the head, because a top table that blocks a service door gets moved on the day whatever your plan says.
Settling it on the plan
It helps to see the row seated before you settle it. Add a top table or a sweetheart table, press Auto-seat, and every other guest is placed around it with your rules kept. Then move anyone you like onto the top table or off it by hand, and they stay where you put them even if you seat the room again.