Learn Farsi: ضخامت اکثر قلم‌نی‌ها مناسب است. - The thickness of most reed pens is suitable.

Unlike ba'zi (some) and besyâri (many) which typically use the preposition az (from/of) to make a partitive phrase, aksar (most) usually connects directly to the following noun using the ezafe vowel (-e). So, "most of the reed pens" is simply aksar-e qalam-ney-hâ.

Where are the hidden ezafe connections in the first part of this sentence?

There is a chain of two ezafe connections: zekhâmat-e aksar-e qalam-ney-hâ. The first -e acts like "of" (thickness of), and the second -e connects the quantifier aksar to the noun it modifies (most of the reed pens).

Why is the plural suffix -hâ placed at the very end of qalam-ney?

Qalam-ney is a compound noun made of qalam (pen) and ney (reed or cane), referring to a traditional Persian calligraphy pen. Because it is treated as one single vocabulary item, the plural suffix -hâ attaches to the very end rather than pluralizing the individual parts.

We are talking about multiple pens, so why is the verb ast (is) singular?

The grammatical subject of the sentence is zekhâmat (thickness), which is a singular noun. Even if the subject were the pens themselves, remember that Farsi generally uses singular verbs for inanimate plural objects anyway!