Nginx itself shouldn’t have any limitations on the number of Node apps you proxy through it. For each app, you’ll just need an Nginx server block similar to this:

server {
    listen 80;

    server_name subdomain1.domain.com;

    location / {
        proxy_pass http://localhost:{YOUR_PORT};
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection 'upgrade';
        proxy_set_header Host $host;
        proxy_cache_bypass $http_upgrade;
    }
}

The main concern would be the general load on the server and the fact that by putting them all on one host, one app could potentially bring the rest down. Generally, there are a number of benefits to breaking separate services out onto separate hosts. Being able to scale up a single service on demand can be useful. One exception to that is that Node is a single-threaded process. So unless you are spawning some kind of worker processes, running a single node app on a multi-core server will lead to underused resources.