Database for 1m users at once

I want to know, if database hosting is the right for me. Normally i have smaller apps, but with this app i will get >20k users with each paying user. And all users in my app will act in the same time.

Example: You have an app for a tv show. Let say “American Idol”. But the show have no jury. The people (>1m users) can vote with the app. All the data is written in realtime to database for later history, how they have voted.

Question: Is it possible to scale the database, that even when i have 1m or maybe 100m users who all acting in db at the same time, will not wait long for data transaction and the database will not crash?

I will start simple with <100k users to save money, but be able to scale when the next customer will use my app and bring his watchers to my app.