If you've got tons of space and want to save money, you can have a horizontal system, you'd ideally want about 1.5 meters of pipe length for every square meter you're heating, according to the companies selling heat pumps. The pipe should be a meter deep and I think it should be a meter from any other section of pipe. Think of the good ol' mobile game Snake.

If you've got tons of money or just don't have the space, you can do a vertical system. It's even more efficient, but you'd want roughly the same amount of meters, in the form of deep holes in the ground - for a bigger house, it's OK to use multiple, just keep a bit of distance between them (6 meters is supposedly OK).

When the air goes cold and it's hard to cool down the outside air to heat up the inside of your home, the ground stays warm so the efficiency drops much less. With the vertical system in particular, there's no measurable drop, since a lot of your refrigerant is running so deep underground, it'd take an ice age to cool down the dirt down there.