A thorough answer would occupy volumes as this article shows: Angels | New Advent. The following serves as an introduction
A good summary and introduction is found in the Catechism of the catholic Church 328-336. In part (sans footnotes)
I. THE ANGELS
The existence of angels - a truth of faith
328 The existence of the spiritual, non-corporeal beings that Sacred Scripture usually calls "angels" is a truth of faith. The
witness of Scripture is as clear as the unanimity of Tradition.
Who are they?
329 St. Augustine says: "'Angel' is the name of their office, not of their nature. If you seek the name of their nature, it is 'spirit';
if you seek the name of their office, it is 'angel': from what they
are, 'spirit', from what they do, 'angel.'" With their whole beings
the angels are servants and messengers of God. Because they "always
behold the face of my Father who is in heaven" they are the "mighty
ones who do his word, hearkening to the voice of his word".
330 As purely spiritual creatures angels have intelligence and will: they are personal and immortal creatures, surpassing in
perfection all visible creatures, as the splendor of their glory bears
This answers but two of your bullet points
- Where they will be? I do not understand. Their abode is the heavens but they go wherever God's want them to accomplish his will which is done in heaven.
- How many are there?
I quote from the New Advent article
The number of angels
The number of the angels is frequently stated as prodigious (Daniel
7:10; Apocalypse 5:11; Psalm 67:18; Matthew 26:53). From the use of
the word host (sabaoth) as a synonym for the heavenly army it is hard
to resist the impression that the term "Lord of Hosts" refers to God's
Supreme command of the angelic multitude (cf. Deuteronomy 33:2; 32:43;
Septuagint). The Fathers see a reference to the relative numbers of
men and angels in the parable of the hundred sheep (Luke 15:1-3),
though this may seem fanciful. The Scholastics, again, following the
treatise "De Coelesti Hierarchia" of St. Denis, regard the
preponderance of numbers as a necessary perfection of the angelic host
(cf. St. Thomas, Summa Theologica I:1:3).