Starting in 1.4, it is possible to get the number of direct replies to a comment without making a separate API call. Here's how.
In the NuGet Package Manager console:
Install-Package Reddit
using Reddit;
...
var reddit = new RedditClient("YourRedditAppID", "YourBotUserRefreshToken");
// Get the number of direct replies to the top comment on the top post of r/AskReddit. --Kris
int? numReplies = reddit.Subreddit("AskReddit").Posts.Top[0].Comments.Top[0].NumReplies;