How many mL of sparkling mineral water should I add to wine to make it adequately sparkling like champagne/bubbly?

You just don't mix water in wine for what I know. Why on earth would you try to do that?
Bullify it in a SodaStream instead (put a little less wine than you usually put water in the bottle).
- You can mix carbonated water and wine - that's called Weinschorle in German
  
  Guess schlorle means something awful ;-) /jk
  Why do you do it? To have a less strong beverage?
  
  Yay new (german compound) word
- i actually kinda like mixing wine and water, gives a light grapy drink
- Sure you can

Bigclive on YT has done some experiments with running drinks through a Sodastream, this seems relevant for you:
https://youtu.be/5-inafP-3zw

Schorle is usually made half and half. You can also go for 1/3 if you have really sparkly water.

The only truly accurate answer is: enough.

You can get a DrinkMate which can carbonate anything, no need to dilute. It has a built in pressure release, and is easily cleanable by design.
Don't use a sodastream, it's not meant for anything but water. It'd make a mess and be very hard to clean.

32.46 ml of soda for each 154.6ml of wine any more and the drink is ruined and you have to throw out the whole bottle. Source and bartender
- Excellent 🫱🫲

Ideally, give folks a chance to mix each pour to taste if you’re sharing this. Follow a wine spritzer recipe for guidelines.
If you want to make it bubbly without diluting, force-carbonating it with CO2 like a homemade soda is fun. Transfer the wine to an appropriate pressure resistant vessel before gassing. Some bargain bubbly is made this way.
Cheap sparklers like cava or cremant de Limoux that use the champagne method might make you happier in a lot of cases, of course. Killer picnic pairings if you’re having fried chicken.
- Is there actually any issue with carbonating non-pure-water stuff with a SodaStream or do they just say that to increase their own mix sales?
  
  Yes, there are issues. I speak from personal experience. That is why I would advise taking a look at Drinkmate instead. Not that they anre any kind of superior company morally, and I do not shill for them, but they can carbonate any beverage, whereas Sodastream definitely cannot, and I have run into issues ignoring that rule and trying to do it myself anyway. Now I use Drinkmate.
  
  My understanding is many of the ones that are only rated for water have additional nooks and crannies where the liquid you are carbonating can get trapped, yielding nastiness over time.
  
  Carbonation adds bubbles. The bubbles in water dissipate very quickly, whereas bubbles in other drinks with sugars or other compounds tend to have a bit more stability in the bubbles (think about how long the foam takes to settle from a beer being poured).
  So when you force carbonate in a bottle, the bubbles tend to rise to the top, and when the pressure is released, will spray out it every direction, causing a huge mess.
  It can be managed by being very slow and methodical at adding pressure, and, more importantly, by being slow and methodical and slowly releasing the pressure. But you have to be ready for it to make a mess.
  
  If there is an argument, I’d hope it’s more about not ending up with unsafe or disappointing chemistry. Suspending CO2 in a liquid doesn’t need anything so proprietary, and especially not from a company that’s an active participant in settler-colonial occupation of Palestine like Sodastream.

Why would you ask this rather than adding it to wine yourself to see what amount you found satisfying?
- Why so quick to assume I havent been doing that in parallel?