The words to make "Eu nu am decât o riglă în penarul meu." are not offered in the word pick system.
What's wrong with "Eu am numai o rigla in penarul meu"? What is the different between doar and numai?
There's nothing wrong with using "numai". They're synonyms.
why "nu" ?
You could think of it this way "I don't have more than one ruler in my pencil case."