the App Store offered more than one million apps by Apple and third parties and is ranked as the world's second largest mobile software distribution network of its kind (by number of currently available applications). There are eight generations of iPhone models, each accompanied by one of the eight major releases of iOS. (original)

Question:
1. The "offered" is a main verb , and so the "is" is?
2. What does the "it" from the "of its kind" refer to? the "App store" or the "software distribution"?
3. What is the meaning of the "by number of currently available applications" ? especially the "by"?
4. Why does the original occur with the "third parties" (plural)?
I think of it as the "third party"(singular)

Question

the App Store offered more than one million apps by Apple and third parties and is ranked as the world's second largest mobile software distribution network of its kind (by number of currently available applications). There are eight generations of iPhone models, each accompanied by one of the eight major releases of iOS. (original)

Question: 
1. The "offered" is a main verb , and so the "is" is?
2. What does the "it"  from the "of its kind" refer to? the "App store" or  the "software distribution"?
3.  What is the meaning of the "by number of currently available applications" ? especially the "by"?
4. Why does the original occur with the "third parties" (plural)? 
 I think of it as the "third party"(singular)

Answer

1. Yes! "offered" and "is" are both main verbs in the sentence.

2. The “it” in “of its kind” refers to the “software distribution network.”

3. The "by" here indicates that the ranking is based on the count of the currently available applications.

4. “Third parties” is used in plural because it refers to multiple entities or developers other than Apple that offer apps on the App Store. “Third party” (singular) would refer to just one such entity, whereas “third parties” encompasses all non-Apple developers collectively.

I hope it helps!

Answer

I have just a few things to add to what Johnny said:

#3: "By number..." is an adverbial phrase that modifies "ranked".  If you place it directly next to the word that it modifies, the sentence becomes more clear:
"It is ranked, by number of currently available applications, as the world second..."

#2: "It" = "the App Store".  The words "mobile software distribution network" are simply a description of the App Store.  If you omit those words, the meaning of the sentence changes very little:
"the App Store offered more than one million apps by Apple and third parties and is ranked as the world's second largest of its kind"