To a certain extent, you'll have very similar outdoor seating choices as you do for indoor seating! Couches, armchairs, loveseats, and chairs are all common options. The specific configuration of chairs and seats you'll need to pick will depend on how much space you have and the number of people you'd like to be able to host at any given time.
Some popular seating types for patio conversation sets are:
Minimal patio conversation sets that usually contain two armchairs and one coffee table in between. These usually only take up a few feet of space, so they're a good choice if you only have a little bit of room to work with.
Three or Four Person Sets
With the addition of a loveseat, you can turn a two-person set into a four-person one! Generally, including two armchairs and a love seat, the three-four person set can also just include two love seats.
The idea is to have the people in your conversation looking at each other, which can be accomplished best in a semi-circular or oval form---thus, two armchairs, a small table, and a love seat are great for up to four people.
Five or Six Person Sets
A 5-6 person patio conversation set will include the same two armchairs, a larger coffee table (an elongated rectangle, instead of a square) and one 3-4 person couch, instead of a loveseat.
This takes up more room than the average smaller conversation set; but it also racks up the comfortability by a large factor. Consider buying a 5-6 person conversation set, even if you don't plan on hosting that many people if you like the idea of lounging on a large couch outdoors!
Seven, Eight, Nine, or More Person Sets
This is where things start to get interesting! You can simply decide to purchase a set with another couch, or with more armchairs, to create your own larger conversation set. This has certain advantages; namely, you can always add more, or replace individual pieces easily if you're ever in need of it.
However, you could also take the route of ordering a gigantic multi-seater sectional, with three or even four sides, with seating capacity for as many as 10. These are more expensive and less flexible, but very luxe and very comfortable.
Of course, you are free to work with the amount of space you have and your personal needs. If you only have a few square feet, you may want to consider one simple loveseat, or one large armchair, just to accommodate one or two people. If you're blessed with a large outdoor entertaining area, on the other hand, you may find it best to create multiple conversation areas, as having one large conversation circle will be cumbersome and less efficient than is likely desired.
When you have a larger yard and wish to engage a large number of people in one conversation, of course, you can move the furniture as needed; but on a more permanent basis, it may be better to have a few different conversational sets for aesthetic purposes.
The specific makeup of your furniture will depend on the style you end up choosing, as we'll discuss in a moment: for example, your armchairs may be all upholstered, might have legs or just be woven wicker to the floor. Each has its advantages and appeal! You'll want to carefully consider the look and feel you're going for before you purchase.