See CCCVII (307) Section of Van Parv for the information that Adhirath had many sons himself after he had got Karn. Of course they were not his uterine brothers, not even half brothers, because Adhirath and Raadhaa were not his real parents.
But their number is not given.

Did Karn marry Eklavya's sister?

Is there any references cited for this?

When in His Life Does Karn Meet Eklavya?

What is the Karn's Period of Study Under Parashuraam
At what point in the chronology of his life, Karn went to study to Parashuraam? We have been unable to find this information in the Mahaabhaarat. Also, the reason traditionally given for Karn's approaching Parahuraam in the guise of a Braahman is that the sage was unwilling to accept non-Braahman (especially Kshatriya) as is students. But this doesn't seem to be stated explicitly in either of the 2 places that we know of that give us any details of Karn's period of study under Parashuraam:

(1) One of the Karn Parv chapters, Chapter 42, Shlok 6-7, describing the lengthy war of words between Karn and Shalya (Karn Parv Chapter 42 in Ganguli's translation) Karn Parv : Section 29 of the Critical Edition; Section 41 of the edition provided in the Sanskrit e-book, "Shaastraani", produced by the Ramakrishna Mission Aashram. Karn has just told Shalya that he studied under Parashuraam in the guise of a Braahman to obtain divine weapons, as well as the incident with the worm.

Mahabharat, 8/42/6-7 - “(He) asked (me who I really was) and I said to the great seer, ‘I am a Soot'. He then cursed me, saying, "Soot, because of the deception through which you have obtained this weapon, it will not, in your time of need, when the time of your death approaches, appear to you by inward light! Indeed, Brahmaa cannot stay in a non-Braahman!' (Thus,) I have forgotten that exceedingly effective weapon in this terrible, tumultuous battle, Sir.” Would I be correct in deducing from this that he said he was a Braahman because he would not otherwise have been taught the use of the Brahmaa weapon? This would seem to be further reinforced by the following:

(2) The Shaanti Parv, 2-3 chapters where Naarad narrates some of Karn's back story to Yudhishthir, (Shaanti Parv: Section 2 of both the Critical Edition and the "Shastraani Edition" - "Karn has just asked Drone to teach him the Brahmaa missile so he can fight Arjun, but Drone, because he's partial to Arjun and knowing Karn's nature, replies:

Shlok 13 - "None but a Braahman who has observed all vows may know the Brahmaa weapon, or a Kshatriya who has practiced penance; no other." Again, would I be right in deducing from this that it must have seemed to Karn that his birth prevented him from learning the Brahmaa missile, so his options were either pretend to be a Braahman or a Kshatriya?

After the incident with the worm, when Karn revealed his true identity to Parashuraam, who then angrily says: 30-31. "Fool, because you have acted deceitfully here from (your) greed for weapons, hence, this Brahmaa weapon shall not appear to you by inward light at the time (you) are slain when you see a warrior equal to you! Brahmaa cannot ever stay fixed in a non-Braahman."

(3) Shaanti Parv : 3/30-31 of both the Critical Edition and the "Shaastraani Edition" After the incident with the worm, Karn has just revealed his true identity to Parashuraam, who then angrily says - "Fool, because you have acted deceitfully here from (your) greed for weapons, hence, this Brahmaa weapon (Brahmaastra) shall not appear to you by inward light at the time (you) are slain when you see a warrior equal to you! Brahmaa cannot ever stay fixed in a non-Braahman!" Would we be right in reaching the same conclusion? Also, what are we to make from the difference in the pronouncements of Parashuraam and Drone?
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So when was the time period when he was educated by Parashuraam?