Is this MEAN stack design-pattern suitable at the 1,000-10,000 user scale?

Let's say that when a user logs into a webapp, he sees a list of information.

Let's say that list of information is served by one of two dynos (via heroku), but that the list of information originates from a single mongo database (i.e., the nodejs dynos are just passing the mongo information to a user when he logs into the webapp).

Question: Suppose I want to make it possible for a user to both modify and add to that list of information.

At a scale of 1,000-10,000 users, is the following strategy suitable:

1. User modifies/adds to data; HTTP POST sent to one of the two nodejs dynos with the updated data.


2. Dyno (whichever one it may be) takes modification/addition of data and makes a direct query into the mongo database to update the data.


3. Dyno sends confirmation back to the client that the update was successful.

Is this OK? Would I have to likely add more dynos (heroku)? I'm basically worried that if a bunch of users are trying to access a single database at once, it will be slow, or I'm somehow risking corrupting the entire database at the 1,000-10,000 person scale. Is this fear reasonable?