一幢幢 Reduplication，means many and dense of skyscraper （高楼林立） other reduplication like 一栋栋（house or skycraper）， 一棵棵（tree），一片片（leaf），一座座（mountain）

幢 is a classifier for tall buildings...摩天 = a loan word for "modern". 大厦 = muti-storey building. So it means skyscraper... Notice in English, the word is made from "sky" & "scrape" - something that is scrapping the sky. Separated by the Huanpu (Yellow Bank) River, on its opposite bank is a row of skyscrappers.

1. 摩天大厦 means skyscraper and 大厦 means a higher building, a tower or a mansion. 幢幢 is a measure word. In your sentence, it means “a lot of” skyscrapers. 2. 隔着黄浦江的对岸则是一幢幢摩天大厦: The opposite side of Huangpu River is a lot of skyscrapers. (I'm not sure if the sentence is grammatically correct but hope you get the meaning. :)