Is the word fleece when referring to a fleece jacket also the plural form, or is that fleeces?
If you write fleece to mean "a jacket or other garment made from a soft warm fabric with a texture similar to sheep's wool," then the plural is fleeces; if you are asking the plural of fleece jacket, that is fleece jackets.
As fleece is also used when talking of a jacket made of fleece (whatever it is sheep's wool, or the soft warm fabric similar to sheep's wool), there is no need to say fleece jacket.
The plural of "fleece jacket" is "fleece jackets".
In English, unlike some other languages, when we make a noun plural we do not modify any associated adjectives. For example, "one hot day", "two hot days", NOT "two hots days".
If you were using "fleece" as a noun, then the plural would be "fleeces". "We used the fleeces from two sheep to make jackets. This was enough for three fleece jackets."