When ‘a number + measure word’ is used with a relative clause, it tends to appear before the relative clause, not immediately before the noun/noun phrase. It is indefinite and it means ‘(a) certain’. (When it appears immediately before the noun/noun clause, the sentence is still correct but may have a slightly different implication.)
zuó tiān wǒ shōu dào yī gè bà bà jì gěi wǒ de lǐ wù 。
Yesterday I received a present that my father sent me.
zhè shì yī jiàn dà jiā dōu yǐ jīng zhī dào de shì ，nǐ bú yòng shuō le 。
This is a matter (that) everybody already knew about. You don’t have to say it anymore.
 Yip, P. C., Rimmington, D., Xiaoming, Z., & Henson, R. (2009). Basic Chinese: a grammar and workbook. Taylor & Francis.