When buying wallpaper or fabrics it is always optimal to have all your product in one batch. That is because shade variation can appear between batches (and samples).
However sometimes its inevitable for whatever reason that you have to mix a batch up. Occasionally we have customers who don't order enough, or some gets damaged later on (which is why many suppliers recommend keeping a spare roll in the same batch). So mixed batches have to be used.
With some thought (and probably a lot of calculations) mixed batches can be used. For example - in two different rooms. If the rooms are separated with no connecting walls then you will probably never notice the difference. Some people can even get away with it in the same room (as long as its not the same wall).
Below is a photo that illustrated varied shading. You can clearly see that even though the design and colour is the same - the two shades are different. If this was on two opposite walls in the same room - you probably would not notice. But on the same wall it is clear.
When calculating wallpaper it is always better if possible to over calculate than under calculate. It may mean you end up with a roll spare - but this is is better than ending up with a sub-optimal end result.