Farming Simulator 19 allows you to combine semi-trailers. There is no limit in how many of them you want to combine. The only problem is how many of them your tractor or any other vehicle can handle. You can combine only those semi-trailers that have a hook. The most popular semi-trailers are TDK / HDK 302. The first TDK semi-trailer has a fixed suspension, HDK semi-trailers have a rear axle steering. They are more difficult to use but they follow each other perfectly (see the picture above).
To combine semi-trailers, attach them one after another. This creates a set that will allow you to carry a lot of goods.