Subscription.cs

public class Subscription
{
    public Subscription(SubscriptionId id, int maxSubscriptions)
    {
        ...
        Dispatch(new SubscriptionCreated(..., maxSubscriptions));
    }
    
    ...
    
    private IList<SubscriptionUser> _users;
    private int _maxSubscriptions;
    
    public void AddUser(string userId, string name)
    {
        if (_users.Count() >= _maxSubscriptions)
            throw new InvalidOperationException("Maximum number of users reached, cannot add more");
            
        Dispatch(new SubscriptionUserAdded(...));
    }
    
    private void When(SubscriptionCreated @event)
    {
        ...
        _users = new List<SubscriptionUser>();
        _maxSubscriptions = maxSubscriptions;
        ...
    }
    
    private void When(SubscriptionUserAdded @event)
    {
        var user = new SubscriptionUser(...);
        _users.Add(user);
    }
    
}