He must have lost count while adding up the figures, because his total is completely wrong.

lost 是动词 lose 的过去分词形式，lose 的意思是"丢失、失去"。 count 在这里是名词，意思是"计数、计算"。 lost count 是一个固定短语，字面意思是"丢失了计数"，引申为"记错数"或"数不清"。