大量の球体をワンメッシュで作成し、ランダムな位置に並べるスクリプト。

GenerateManySphere.cs

using UnityEngine;

#if UNITY_EDITOR
using UnityEditor;
[CustomEditor(typeof(GenerateManySphere))]
public class GenerateManySphereInspector : Editor
{
    public override void OnInspectorGUI()
    {
        base.OnInspectorGUI();
        var gs = target as GenerateManySphere;
        EditorGUILayout.LabelField("VertexCount", (gs.SphereCount * gs.CountXZ * gs.CountY).ToString());
    }
}
#endif

[RequireComponent(typeof(MeshFilter))]
[RequireComponent(typeof(MeshRenderer))]
public class GenerateManySphere : MonoBehaviour
{
    [SerializeField] bool generateMeshOnUpdate = false;
    [SerializeField] int sphereCount = 50; // 球体の個数
    [SerializeField] int yCount = 12; // Y軸を直径とする円の角度の分割数
    [SerializeField] int xzCount = 12; // XZ平面上での円の角度の分割数
    [SerializeField] float positionRandom = 8f;
    [SerializeField] int randomSeed = 100;
    [SerializeField] float sphereSize = 1f;
    private Mesh mesh;

    public int SphereCount => sphereCount;
    public int CountY => yCount;
    public int CountXZ => xzCount;
    void Start()
    {
        mesh = new Mesh();
        GetComponent<MeshFilter>().mesh = mesh;
        Create();
    }

    void Update()
    {
        if (generateMeshOnUpdate)
        {
            Create();
        }
    }

    void Create()
    {
        if (yCount < 3) { yCount = 3; }
        if (xzCount < 3) { xzCount = 3; }

        Random.InitState(randomSeed);

        int vertexCountXZ = xzCount + 1;

        // 法線・頂点座標の作成
        Vector3[] vertices = new Vector3[sphereCount * vertexCountXZ * yCount]; //頂点座標
        Vector3[] normals = new Vector3[vertices.Length]; // 法線
        Vector2[] uv = new Vector2[vertices.Length];
        int vi = 0;
        for (int si = 0; si < sphereCount; si++)
        {
            Vector3 position = (
                new Vector3(Random.value, Random.value, Random.value) - Vector3.one / 2f                
            ) * positionRandom;
            for (int yi = 0; yi < yCount; yi++)
            {
                float radian1 = yi * Mathf.PI / (yCount - 1) - Mathf.PI / 2f;
                float y = Mathf.Sin(radian1);
                for (int xi = 0; xi < vertexCountXZ; xi++)
                {
                    float radian2 = xi * 2f * Mathf.PI / xzCount;
                    float x = Mathf.Cos(radian1) * Mathf.Cos(radian2);
                    float z = Mathf.Cos(radian1) * Mathf.Sin(radian2);
                    uv[vi] = new Vector2((float)si / sphereCount, 0f);
                    normals[vi] = new Vector3(x, y, z);
                    vertices[vi++] = new Vector3(x, y, z) * sphereSize + position;
                }
            }
        }

        // 頂点インデックスの作成
        int[] triangles = new int[sphereCount * xzCount * (yCount - 1) * 6];
        int ti = 0;
        int triangleOffset = 0;
        for (int si = 0; si < sphereCount; si++)
        {
            for (int i = 0; i < yCount - 1; i++)
            {
                for (int j = 0; j < xzCount; j++)
                {
                    triangles[ti++] = triangleOffset + vertexCountXZ; // 2
                    triangles[ti++] = triangleOffset + 1; // 1
                    triangles[ti++] = triangleOffset + 0; // 0
                    triangles[ti++] = triangleOffset + 1 + vertexCountXZ; // 3
                    triangles[ti++] = triangleOffset + 1; // 1
                    triangles[ti++] = triangleOffset + vertexCountXZ; // 2
                    triangleOffset += 1;
                }
                    triangleOffset += 1;
            }
        }

        // メッシュ作成
        mesh.vertices = vertices;
        mesh.triangles = triangles;
        mesh.normals = normals;
        mesh.uv = uv;
    }
}

大量の球体をワンメッシュで生成します。　頂点数が65536を超えると表示がおかしくなるので注意