Mango (Embe) belongs to the JI/MA class, where all plural words start with "ma". So if you see the word starts with "ma", ex maembe (mangoes) or madirisha (windows), then you know it's plural.
Why is this mangoes and not mango? Why is mangoes not 'waembe'? Never thought I'd have so many questions about large mangoes
Looks like you need to go back several lessons and read the lesson tips for the JI/MA noun class.
It would have been "waembe" if the word mango (embe) belonged to the M/WA-class, like ex teacher (mwalimu/walimu). But now that mango belongs to the JI/MA-class it becomes "maembe", because all the plural words in that class starts with "ma".