From Valheim Wiki
A Workbench is a base structure used for crafting, repairing items and building various structures within 20 meters of it.
They don't require any other structure to be built.
Building structures doesn't require roof or any cover. They must be have a roof and 70% cover for crafting and repairing. Repairing items costs no resources.
Upgrading a workbench enables additional crafting recipes and higher upgrade levels for items. A workbench can be upgraded up to level 5 by building the following structures near it:
total: 4 row(s)
Duplicate upgrades do not stack. These upgrades can be used to support more than one workbench at the same time, as long as they're in range.
Used to craft
total: 58 row(s)