Let's say I'm a Char Lvl15 Sorcerer-Warlock (Sorcerer Lvl 12/Warlock Lvl 3). I've currently maxed out my cantrips known (limit is 8).

I can take a feat at this level. If I choose Spell Sniper which allows me to learn a new cantrip, can I learn a new one even though I've maxed out my cantrips known?

If no, I suppose I'll have to replace an existing cantrip with a new one if I wanted to. 

If yes, does my cantrips learned increase by 1 (i.e., it's now 9)? Or does it not count towards the cantrips known?

Does the same principle work for spells learnt via feats?

Also, do racial spells count towards cantrips known? For example, if I am a Half Elf Drow variant, I know the Dancing Lights cantrip? Does this count towards the cantrips known?

Sorry for all these questions. I'm getting into D&D and I couldn't find any clear answer for my questions in the current rules/books.

Thanks!

Yes, the feat lets you learn a new cantrip. It does not count against the amount you already know.

Each feat will say how it works separately. Some add to your existing pool, others will provide them as separate spells.

Racial spells are separate from your class spells.

Thanks! :)

Creating your character on dndbeyond already knows these rules, so it will automatically tell you how many spells you are still able to memorise separately from those gained through feats and race.

I have a remarkably similar question so I'm going to piggy back on this OP and see how that works for me.

The scenario that I came up with this this:  I have an Oathbreaker Paladin who just achieved an ASI.  They're taking the Shadow Touched (Charisma) Feat.  This lets them learn a 1st level necromancy spell (they're taking Ray of Sickness) and Invisibility. 

Originally, I thought that the Feat offered these spells independently of their spell slots.  However, in re-reading them, it says that they learn these spells and can cast each once per day w/o using a spell slot.  My new interpretation of this is that they can cast each of the spells once per day w/o using a spell slot but, if they want to use a spell slot, they can cast them as often as they want.  Is that congruent with other people's understanding?

@RebelAndARunner The Feat itself answers your question

• "You can cast each of these spells without expending a spell slot...
  ...Once you cast either of these spells in this way, you can’t cast that spell in this way again until you finish a long rest"
• "You can also cast these spells using spell slots you have of the appropriate level. "

So yes, you get 1 free cast per long rest.
And yes, you can choose to cast the spell using spell slots (regardless if you used the free cast or not)

You are correct.  I saw that after I posted the question and thought "Oh, look, it's my RTFM moment".

Thanks!