add limit checks for tag count

src/Discord.Net.Rest/Entities/Channels/ForumHelper.cs

@@ -14,6 +14,9 @@ internal static class ForumHelper
     {
         var args = new ForumChannelProperties();
         func(args);
+
+        Preconditions.AtMost(args.Tags.IsSpecified ? args.Tags.Value.Count() : 0, 5, nameof(args.Tags), "Forum channel can have max 20 tags.");
+        
         var apiArgs = new API.Rest.ModifyForumChannelParams()
         {
             Name = args.Name,

src/Discord.Net.Rest/Entities/Channels/ThreadHelper.cs

@@ -1,3 +1,4 @@
+using Discord.API;
 using Discord.API.Rest;
 using System;
 using System.Collections.Generic;
@@ -51,6 +52,9 @@ namespace Discord.Rest
         {
             var args = new ThreadChannelProperties();
             func(args);
+
+            Preconditions.AtMost(args.AppliedTags.IsSpecified ? args.AppliedTags.Value.Count() : 0, 5, nameof(args.AppliedTags), "Forum post can have max 5 applied tags.");
+
             var apiArgs = new ModifyThreadParams
             {
                 Name = args.Name,
@@ -117,6 +121,7 @@ namespace Discord.Rest
             Preconditions.AtMost(allowedMentions?.RoleIds?.Count ?? 0, 100, nameof(allowedMentions.RoleIds), "A max of 100 role Ids are allowed.");
             Preconditions.AtMost(allowedMentions?.UserIds?.Count ?? 0, 100, nameof(allowedMentions.UserIds), "A max of 100 user Ids are allowed.");
             Preconditions.AtMost(embeds.Length, 10, nameof(embeds), "A max of 10 embeds are allowed.");
+            Preconditions.AtMost(tagIds?.Length ?? 0, 5, nameof(tagIds), "Forum post can have max 5 applied tags.");
 
             // check that user flag and user Id list are exclusive, same with role flag and role Id list
             if (allowedMentions != null && allowedMentions.AllowedTypes.HasValue)
@@ -178,6 +183,8 @@ namespace Discord.Rest
             Preconditions.AtMost(allowedMentions?.RoleIds?.Count ?? 0, 100, nameof(allowedMentions.RoleIds), "A max of 100 role Ids are allowed.");
             Preconditions.AtMost(allowedMentions?.UserIds?.Count ?? 0, 100, nameof(allowedMentions.UserIds), "A max of 100 user Ids are allowed.");
             Preconditions.AtMost(embeds.Length, 10, nameof(embeds), "A max of 10 embeds are allowed.");
+            Preconditions.AtMost(tagIds?.Length ?? 0, 5, nameof(tagIds), "Forum post can have max 5 applied tags.");
+
 
             // check that user flag and user Id list are exclusive, same with role flag and role Id list
             if (allowedMentions != null && allowedMentions.AllowedTypes.HasValue)

src/Discord.Net.Rest/Entities/Guilds/GuildHelper.cs

@@ -351,6 +351,8 @@ namespace Discord.Rest
             var props = new ForumChannelProperties();
             func?.Invoke(props);
 
+            Preconditions.AtMost(props.Tags.IsSpecified ? props.Tags.Value.Count() : 0, 5, nameof(props.Tags), "Forum channel can have max 20 tags.");
+
             var args = new CreateGuildChannelParams(name, ChannelType.Forum)
             {
                 Position = props.Position,