All worlds are numbered by the Lord. The worlds that are not numbered are the telestial, terrestrial and celestial worlds. The "worlds without number" are these three kingdoms.
And worlds without number have I created; and I also created them for mine own purpose; and by the Son I created them, which is mine Only Begotten.
But only an account of this earth, and the inhabitants thereof, give I unto you. For behold, there are many worlds that have passed away by the word of my power. And there are many that now stand, and innumberable are they unto man; but all things are numbered unto me, for they are mine and I know them.
...and are recorded in the book of names of the sanctified, even them of the celestial world.
D&C 76:71, 98, 109
And again, we saw the terrestrial world...
And the glory of the telestial is one, even as the glory of the stars is one; for as one star differs from another star in glory, even so differs one from another in glory in the telestial world;
But behold, and lo, we saw the glory and the inhabitants of the telestial world