"How many brothers and sisters does Marcus have?"
Translation:Quot fratres et sorores habet Marcus?
Quot is only used when you're asking for a "number" in response: "How many?" (So, how many slaves were there / does someone have; how many fields did he plant; how many fish(es) did they buy; etc.)
Quomodo asks about the manner in which something was done: "In what way / How?"
This doesn't work, or make sense, because: "Marcus" has brothers and sisters, so he's the subject (nominative: Marcus ); no one is talking to him, so there's no need for the vocative Marce .
Secondly, there's no need or use for se in this sentence. (Marcus se habet + adverb male "badly" or bene "well" tells us "how Marcus is feeling".)
He's definitely not accusative in this sentence: he's the subject, since the question is how many brothers & sisters HE has .
If we refer to Marcus as HE, he's the subject (of a verb): He has brothers and sisters; He is a great man; He throws fish on the floor, etc.
It's just one of the oddities of an English question ("How many brothers and sisters does Marcus have?") and of Latin word order (Quot frātrēs et sorōrēs habet Marcus ?), that the name "Marcus" can be placed late in the sentence. But what's relevant is what "Marcus" is doing in the particular sentence.
Here, he's "having" or "possessing" brothers and sisters: so, he's the subject of the verb (habet).
Sometimes people think that a subject "has to come first" in a sentence, and that often happens in English, though there's no need for it in Latin:
Vir canem mordet = Man bites dog. In Latin, Canem mordet vir and Mordet canem vir all still mean "man bites dog," since the word for "man," vir , is in the nominative (and the dog, canem , is in the accusative).
Even in English, the subject need not come first:
"And after many a summer dies the swan." The swan is the subject of the verb "dies," even though it comes last in the sentence.
We'll use the accusative form, Marcum , only when someone else is doing something to Marcus: "They throw Marcus ( = HIM) on the floor," or maybe "They throw fish at Marcus" ( = ad Marcum, "at / towards HIM").
You reference the vocative case, Marce , used only when speaking to him directly: Age, Marce! Nōlī piscēs in pavīmentum iacere! (Come on, Marcus! Don't throw fish on the floor!)