Many, Few (多い・少ない)

多い never sits directly in front of a noun the way English “many books” does, 多い本 is wrong. Say 本が多い (books are many) as its own clause, or use たくさんの本 (many books) instead if you need it inside a noun phrase.

多くて is just 多い’s て-form, the same chaining trick every い-adjective uses to link two descriptions into one sentence: レストランが多くていい literally strings together “restaurants are many” and “good” into “has a lot of restaurants, which is nice.” 多い does turn directly into 多い町 (a town with a lot of X) when it’s modifying the noun as a full clause rather than standing alone, that’s different from trying to jam it in front of the counted noun itself.