How many stars are in the universe?
Based on current estimates, there are between 200 - 400 billion stars in our galaxy (The Milky Way). There are possibly 100 billion galaxies in the Universe. So taking the average of our galaxy, gives approximately 3 x 1024 stars. So about 3 septillion. This has been equated to the same number of grains of sand that are on Earth.
One source (BBC) stated that there are 1,000 stars to every grain of sand on Earth!!
There are an estimated 100 to 200 billion galaxies.
So taking a conservative number of 100 billion stars per galaxy, gives an approximate total of 10,000,000,000,000,000,000,000 stars. (which is 10 sextillion)
Hashem told Avraham:
וַיּוֹצֵא אֹתוֹ הַחוּצָה, וַיֹּאמֶר הַבֶּט-נָא הַשָּׁמַיְמָה וּסְפֹר הַכּוֹכָבִים--אִם-תּוּכַל, לִסְפֹּר אֹתָם; וַיֹּאמֶר לוֹ, כֹּה יִהְיֶה זַרְעֶךָAnd He brought him forth abroad, and said: 'Look now toward heaven, and count the stars, if thou be able to count them'; and He said unto him: 'So shall thy seed be.'
We further find these verses in Devarim:
יְהוָה אֱלֹהֵיכֶם, הִרְבָּה אֶתְכֶם; וְהִנְּכֶם הַיּוֹם, כְּכוֹכְבֵי הַשָּׁמַיִם לָרֹבthe LORD your God hath multiplied you, and, behold, ye are this day as the stars of heaven for multitude.--
יְהוָה אֱלֹהֵי אֲבוֹתֵכֶם, יֹסֵף עֲלֵיכֶם כָּכֶם--אֶלֶף פְּעָמִים; וִיבָרֵךְ אֶתְכֶם, כַּאֲשֶׁר דִּבֶּר לָכֶם
The LORD, the God of your fathers, make you a thousand times so many more as ye are, and bless you, as He hath promised you!--
Rashi first asks that 600,000 doesn't even come close to the amount of stars! He answers that the verse means that we're compared to the sun and stars only in that we will exist forever just as they do. However, the רא"ם on that Rashi says that that's true in Moshe's time, but in the future, we will actually be as numerous as the stars, as is hinted to by the word "לָרֹב".
On the next verse, Rashi says:
מהו יוסף עליכם ככם אלף פעמים . מהו
שוב ויברך אתכם כאשר דבר לכם אלא אמרו לו למשה אתה נותן קצבה לברכתנו כבר הבטיח הקב״ה את אברהם (בראשית יג) אשר אם יוכל איש למנות וגו׳ אמר להם זו
משלי היא אבל הוא יברך אתכם כאשר דבר לכם
which means that instead of 10 sextillion, Moshe's promise will make us 10 septillion, according to the conservative estimates. According to the other estimates, instead of 3 septillion, we'll be 3 octillion. And that doesn't include Hashem's additional Beracha, which makes it unlimited.