If you don't know about options for hot, I would base it on cold salads and potentially have some kind of potato side etc that can be hot. Other potentials for hot stuff would be sausage rolls and chicken wings.
A side of salmon, already portioned up - or darnes laid nicely together to look like a side. A few prawns and crab claws scattered around the dish too. Sometimes, there is a bowl of prawn cocktail on the side as well (there are a few who love it!).
A ham, already sliced up. A platter of roasted chicken drumsticks. Perhaps a nice quiche (vegetarian would be nice - or 2 including 1 veggie) - which could be hot/cold.
Nice bowls of salad leaves - I would do a mixed bowl and another of plain lettuce (either iceberg or butterhead).
A bowl of cherry tomatoes. A bowl of olives.
Tomato, cucumber and feta salad.
Potato salad. Coleslaw. Both must haves.
Devilled eggs (?). Hard boiled eggs, halved, laid on flat side, with small dollop nice mayo on top and tiny scattering of cayenne/paprika.
A pasta or a rice salad. Perhaps something like broccoli with roasted hazelnuts. Sweetcorn with mixed beans.
Overall, for 20 people, in our family we'd do maybe 4-5 different salads but have a couple of bowls of the big favourites.
A few of those salads would already be dressed, and we tend to have things like proper salad cream (like Heinz but an aunt makes it with fresh cream and vinegar), a vinagrette, seafood sauce (if not on the prawns already), mustard (grainy and English both required for different people on their ham!) and maybe something like a blue cheese for the chicken. Just scattered about the table (some near specific things, others just scattered).
Lots of fresh crunchy French stick already cut, with soft butter beside it (depending on numbers, we may butter it in advance - we've had some pretty big gatherings in 1 house).
In our house and with neighbours, there's always a cheese board, but the bigger family gatherings would not be great lovers of cheese - but that can be nice to have out if your diners would eat it. And easy to put out once salads are cleared off, just lay out cheese, baskets of crackers and bowls of grapes.
Afters is usually large bowls of fresh fruit salad, jelly, and something like meringue roulades or raspberry mousse, or maybe chocolate mousse. With bowl of cream on the side (and ice creams for kids - multi packs of them rather than scooping out tubs when larger numbers involved, but generally a tub if "only" ourselves (the 20) as some adults like a scoop too!!
A big bowl of lemon slices and another of ice for drinks. Lots of soft drinks.
Cutlery wrapped in paper napkins in a basket.
And a large box of chocs for passing around with tea/coffee later.
I know not a lot there that can be frozen ahead, but it's what we've found works.