This is good as a rule of thumb. A more detailed analysis may be found here:
For ett-words that end in in a consonant, the indefinite singular and plural are the same. Ett djur, två djur. However, in the singular use, it will often be preceded by de indefinite article ett. Furthermore, the number of an ett-word may sometimes be deduced from words that describe it. "Vårt djur" = "our animal", but "våra djur" = "our animals".