I am following a sauerkraut recipe which calls for a specific amount of salt. They say I can add other vegetables, like carrots. Do I need more salt if I add more vegetables?
If you are fermenting sauerkraut, then you should keep the salt content the same if you add other vegetables. The salt helps keeping undesirable bacteria in check, so you probably don't want to "dilute" it. Increase the amount of salt so the ratio between cabbage/vegetables and salt stays the same.
TL;DR: Treat any additional vegetables like cabbage when calculating the amount of salt.
The salt amount usage will depends to the weight of the vegetables and water.
For example, if your current recipe calls for 2 lb. of cabbage, but now you are planing to do less cabbage and substitute that amount with carrots, then you don't need more salt.
However, if you would use 2 lb. of cabbage (according to recipe) and wish to put 1 lb. carrots into your mix, then naturally, you will need more salt, since you have to use more water to cover the additional vegetables.