using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {

            Console.WriteLine("vvedit a");
            int a = int.Parse(Console.ReadLine());
           if((1<=a)&&(a<=4)) Console.WriteLine("few") ;
      else if((5<=a)&&(a<=9)) Console.WriteLine("several");
           else if ((10 <= a) && (a <= 19)) Console.WriteLine("pack");
           else if ((20 <= a) && (a <= 49)) Console.WriteLine("lots");
           else if ((50 <= a) && (a <= 99)) Console.WriteLine("horde");
           else if ((100 <= a) && (a <= 249)) Console.WriteLine("throng");
           else if ((250 <= a) && (a <= 499)) Console.WriteLine("swarm");
           else if ((500 <= a) && (a <= 999)) Console.WriteLine("zounds");
           else if ((1000 <= a) && (a <= 2000)) Console.WriteLine("legion");


           }
        }
    }

See on your selection statements. variable cannot be less then one and four at once.
You maybe had in mind (1>=a)&(a<=4)