The short answer is 'several people'. Nahash, coming from a root
word meaning 'serpent', was a unisex name; like 'Lesley' it could
be either a boy or girl's name. In the ancient Levant (Bible lands
of Palestine), the bible mentions 2 specific persons in 2nd Samuel
chapters 10-12.
The first Nahash was King of Ammon, and suffered a minor defeat
by King Saul; two of his sons eventually succeeded him during the
reign of King David (1077-1037 BC). The second Nahash was the
parent of David's two older half-sisters, Zeruiah and Abigail. If
that Nahash was female, she was David's mother, if male, the former
husband of David's mother (his father, of course, was Jesse).
Scholars generally believe Nahash was the (possibly) non-Israelite
father of David's half-sisters and that his mother was a widow with
two young daughters when she married Jesse.
The text of the Bible is ambiguous on the second Nahash's
gender, perhaps deliberately so. Later on, some scholars (with no
evidence but considerable vehemence) insisted Nahash was another
name of Jesse, which is highly unlikely.
However, it is possible that as an enemy of Saul, David was
given shelter by King Nahash, despite the Ammonites technically
being considered reprehensible by the ancient Israelites. As today,
names tended to be familial, tribal, national; perhaps David got a
warm welcome from the Ammonite king because his two half-sisters
were Nahash's relatives-possibly as close as nieces or
granddaughters through a dead namesake son. Nahash wasn't a popular
Hebrew name, though not entirely unheard of.
It would have been highly inconvenient for later Rabbis of the
more radical type to have it known that the glorious King David's
mother a) was married to someone else before Jesse, b) that her
first husband was a non-Jew, c) that he belonged to the despised
Ammonites and d) was probably/possibly a member of the Ammon Royal
Family.
Of course that theory is just conjecture, but it could account
for the deliberate ambiguity that David's family is treated with in
the Bible, which is unusual given that usually great clarity was
utilised to establish relationships.
For example, David was born in 1107 BC, and was only 14 when he
first encountered Saul and Jonathan, in 1094-3BC. Although he and
Jonathan became great friends, they did not even register each
other's existence until David became a semi-permanent member of
Saul's royal retinue 18 months later in 1092 BC.
The point is however that in 1194 BC, David's father Jesse was
"already old amongst men" (1st Samuel 17:12). Since the Israelites
were accustomed to persons living lifespans of 70-80 years, Jesse
must have been at least 85 years old in 1192 BC for his lifespan to
have been so noteworthy.
In short, Jesse was either not monogamous, or he had been
widowed at least once (like David's mother) and David's mother had
to be at least 40 years younger that Jesse (b.abt 1180-1175 BC) to
have been of childbearing age in 1107, when she had David. Given
she already had two young daughters, it is likely she was probably
in her late 20s/early 30s in 1107 bC.
It is therefore highly likely that at least Jesse's 3 eldest
sons, Eliab, Abinadab and Shimmea, if not more, were also
half-brothers of David, by an earlier or the primary wife of Jesse.
Altogether he had 8 sons, though the 7th born died without any
children and is unnamed in the bible.
As a man "mighty in wealth", Jesse might have been tempted to
have wives and concubines, and he was also unlikely to have many
rivals for his fortune; his father Obed was the son of Boaz (an
extremely rich Israelite) and Ruth the Moabitess - given Boaz was
in his 60s-70s when Obed was born, it is highly probable Obed was
an only child who inherited his father's entire fortune. 1st Samuel
17 highlights the 'bad blood' between David and Eliab, Jesse's
firstborn and heir, making it more likely they were half-brothers.
David certainly had a great deal more to do with his half-sisters'
sons during his reign, indicating a lack of emotional closeness to
Jesse's other sons.
It is possible, though no evidence exists, that Jesse married
consecutively rather than concurrently - his first 3 sons could
have been by his first wife; sons 4,5 and 6 by his second wife or a
simultaneous wife or concubine, then sons 7 and 8 could have been
by David's mother; perhaps David's only full brother was the 7th,
immediately his elder, who died as a young man without children.
There is no way to know.
Finally, one possibility to really confuse - it is entirely
possible that Nahash was a title rather than a personal name, like
Pharaoh, and Abimelech and Jabin were titles rather than names. A
more modern parallel, though fanciful, is that of Merlin and Arthur
- Merlin, the ancient name for Eagle was a title, not a name, and
so was Arthur, deriving from a title meaning 'Bear'; in ancient
tribes, names had great significance, so calling a great warrior or
wise man (magician) Nahash (snake like cunning) or the Bear
(Arthur) was a bit of propaganda. Again, there is no way to know
due to the immense span of time since that period, but it is
interesting to speculate on the deliberate ambiguity that seems to
exist in the text on the issue of who Nahash really was.
