There are few mistakes in your composition.
FYI: you can omit は from 作文にはほとんど. 作文にはほとんど… with は implies that compositions of the others have many mistakes, but not in yours. は implicitly highlights the gap between the main topic and unmentioned counterparts.
“A few mistakes” and “few mistakes” are basically translated in Japanese as:
君の作文に間違いはほとんどありません。= (lit.) There are almost no mistakes in your compositions.
若干の and 多少の are interchangeable in the first sentence.