Help Npc Skill Seler

Who can help me add to this npc skill seller the restriction that a plaque can not buy more than 6 skill

package com.l2jfrozen.gameserver.model.actor.instance;

import com.l2jfrozen.gameserver.datatables.SkillTable;
import com.l2jfrozen.gameserver.model.L2Skill;
import com.l2jfrozen.gameserver.network.serverpackets.NpcHtmlMessage;
import com.l2jfrozen.gameserver.templates.L2NpcTemplate;
/*
 * @author Rizlaaa
 * 
 * @editor Gabriel Fleck
 * @Date 2018/12/15
 * @version 1.1
 */
public class L2SkillSellerInstance extends L2NpcInstance
{
    public final static int ITEM_ID = 3470;
    public final static int ITEM_COUNT = 20;

buy restriction skill = max 6

    private final int[] SKILL_IDS =
    {
        3124, 3125, 3132, 3133, 
        3134, 3135, 3136, 3137, 
        3138, 3139, 3140, 3141
    };
        
    public L2SkillSellerInstance(int objectId, L2NpcTemplate template)
    {
        super(objectId, template);
    }
    
    @Override
    public void showChatWindow(L2PcInstance j, int val)
    {
        if (j == null)
            return;
        StringBuilder t = new StringBuilder();
        NpcHtmlMessage n = new NpcHtmlMessage(getObjectId());
        sendHtml(t, n, j);
    }
        
    private void sendHtml(StringBuilder t, NpcHtmlMessage n, L2PcInstance j)
    {
        t.append("<html><body><center>");
        t.append("Hello, do you want some special skills?<br1>");
        t.append("Choose whatever you want but don't forget<br1>");
        t.append("you need 20 Gold Bars for each one.<br>");
        for (int i : SKILL_IDS)
        {
            L2Skill s = SkillTable.getInstance().getInfo(i, 10);
            String name = "";
            if (s != null)
                name = s.getName();
            if (name != "")
                t.append("<center><a action=\"bypass -h skill" + i + "\">" + name + " Lv 10</a><br1>");
        }
        t.append("</center></body></html>");
        n.setHtml(t.toString());
        j.sendPacket(n);
    }
}

please beautifully helped as a little bit kicking this restriction code