How do I limit the number of elements iterated over in a foreach loop?

just iterate over the top 6 from the collection:

foreach(var rssItem in rss.Channel.Items.Take(6))

Not to be too obvious but...

int max = Math.Min(6, rss.Channel.Items.Count);
for (int i = 0; i < max; i++)
{
   var rssItem = rss.Channel.Items[i];
   //...
}

I know it's old school, and not filled with all sorts of Extension method goodness, but sometimes the old school still works... especially if you're still using .NET 2.0.

Use Enumerable.Take:

foreach(var rssItem in rss.Channel.Items.Take(6)) {
    // go time!
}

Note that

rss.Channel.Items.Take(6)

does not do anything except instantiate an implementation of IEnumerable that can be iterated over to produce the first six items in the enumeration. This is the deferred execution feature of LINQ to Objects.

Note further that this assumes .NET 3.5. If you are working with an earlier version of .NET, you could use something along the lines of the following:

static IEnumerable<T> Take<T>(IEnumerable<T> source, int take) {
    if (source == null) {
        throw new ArgumentNullException("source");
    }
    if (take < 0) {
        throw new ArgumentOutOfRangeException("take");
    }
    if (take == 0) {
        yield break;
    }
    int count = 0;
    foreach (T item in source) {
        count++;
        yield return item;
        if (count >= take) {
            yield break;
        }
    }
}

Then:

foreach(var rssItem in Take(rss.Channel.Items, 6)) {
    // go time!
}

This assumes .NET 2.0. If you're not using .NET 2.0 you should seriously consider upgrading.

You could also just break out of the loop if you don't want to use linq.

        int count = 0;
        foreach (var rssItem in rss.Channel.Items)
        {
            if (++count == 6) break;
            ...
        }

If you're interested in a condition (i.e. ordering by date created)

foreach(var rssItem in rss.Channel.Items.OrderByDescending(x=>x.CreateDate).Take(6)) 
{
//do something
}

Perhaps if you want to get those created by a certain user, with that same sort

foreach(var rssItem in rss.Channel.Items
                          .Where(x=>x.UserID == 1)
                          .OrderByDescending(x=>x.CreateDate)
                          .Take(6)) 
{
//do something
}

rss.Channel.Items.Take(6)