I lost count of how often I heard this sentence when I worked for a drug company.

lost 是动词 lose 的过去式，意思是"失去、遗忘"。 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"或"数不过来"。