What Storm Raven said.
It's very, very heavily dependent upon the culture and the society.
The unhelpful answer is: Whatever you want it to be
The more honest answer is: What RL society are you, however loosely, modelling your world on?
The British nobility was /is very small compared to the rest of Europe.
You are talking about 600 people or so with actual titles, below that you have a large, disparate group of people who are gentlefolk but aren't formally nobility.
Pre-revolution France had a noble class of somewhere in the 100,000+ range.
However, by no means all of those spent their time hanging out at Versailles.
A great many of them were often no richer than the peasents they lived amongst - but they did have the right to carry a sword; certain tax advantages and the exlcusive right to become army officers.
I believe in Italy inheritence was partible and you could have multiple people holding the same title - this can get confusing.
It's get even better when you consider that in France a title often goes with a property - so to become the Marquis de "X" you have to buy the Chateau de "X".
You can make this as complicated as you want. Descendents and relatives are normally considered to be gentle, if not noble - a lot however depends on how much money they have and what, if anything, they do for a living.