I lost count of how often I heard that phrase at my old job.

lost 是动词 lose 的过去式，意思是"失去、丢失"。 count 是名词，意思是"计数、总数"。 lost count 是一个固定短语，字面意思是"丢失了计数"，引申为"记不清数量、无法准确计算"。