上百万 is a quantity expression meaning 'several million' or 'over a million'. It is typically used as an attributive before a noun (e.g. 上百万人口 'several million people'). The word is not a standard measure word (m) — its POS is likely a number+classifier phrase or an attributive numeral. Note that the pinyin in this entry (shǎng) appears incorrect; the standard pronunciation is shàng bǎi wàn.