As far, as I know, there's no legal definition concerning the amount of pallets stacked, or load height.
Only rule which applies here is to maintain the truck stability (its capacity and its reduction in accordance to load centre distance).
Sometimes in the standards or safety documentation exists a phrase "use the additional means to secure..."
It means that in case of unstable loads, some safety device preventing the collapse should be used. Like load stabilzers or load backrests.
Load backrest is not an obligatory/standard attachment, pretty often the top manufacturers treat it as on option (see Linde).