particles/MainMenu/MenuBackground.cs

using Godot;
using System;

public class MenuBackground : Node2D
{

    private Node2D _particleNodes;

    public override void _Ready()
    {
        //_particleNodes = GetNode<Node2D>("ParticlesNodes");
        //var particleScene = GD.Load<PackedScene>("res://ParticleSimulation/ParticleNode.tscn");
        //var particleNode = particleScene.Instance<ParticleNode>();
        //_particleNodes.AddChild(particleNode);
    }

    public void SetParticleCount(int nParticles)
    {
        int currentCount = _particleNodes.GetChildCount();
        if (currentCount < nParticles)
        {
            int addCount = nParticles - currentCount;
            for (int i = 0; i < addCount; i++)
            {
                CreateParticle();
            }
        }
        // TODO pick random particles to delete
    }

    private void CreateParticle()
    {
        var particlePackedScene = GD.Load<PackedScene>("res://ParticleSimulation/ParticleNode.tscn");
        var particleNode = particlePackedScene.Instance<ParticleNode>();
        _particleNodes.AddChild(particleNode);
        var screenSize = GetViewportRect().Size;
        var position = new Vector2(
            (float) GD.RandRange(0, screenSize.x),
            (float) GD.RandRange(0, screenSize.y));
        particleNode.Position = position;
    }

        public override void _Process(float delta)
    {
        //var particleNodesArray = _particleNodes.GetChildren();
        //foreach (ParticleNode particle in particleNodesArray)
        //{
        //    particle.Position += new Vector2(0.1f, 0f);
        //}
    }
}