I lost count of how often I heard this sentence when I used to work for a drug company.

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"数不清、记不清具体数字"的意思。