If you just need one sign-in account, then it will be billed as a single agent account. Most of these services define users by the number of different sign-ins.
However, if you just need a knowledge base to start with, you might try the free Sprout account level with our top pick Freshdesk. It doesn’t offer the full help desk suite, but does allow unlimited user access to knowledge base functions. Plus you can move your account to higher level plans with more help desk features if/when when you need them. You can see how it stacks up to Freshdesk’s other plans here.