Nobody knows how many are going to heaven or hell, aside from God.
Many saints and early church fathers to believe that the number of the saved is few in comparison to the damned, but at least for the latter this view may have resulted from a misguided interpretation of scripture.
On the other hand,
[quote=“Catholic Encyclopedia: Predestination”]Pointing to several texts of the Bible (Matthew 7:14; 22:14) and to sayings of great spiritual doctors, the rigorists defend as probable the thesis that not only most Christians but also most Catholics are doomed to eternal damnation. Almost repulsive in its tone is Massillon’s sermon on the small number of the elect. Yet even St. Thomas (loc. cit., a. 7) asserted: “Pauciores sunt qui salvantur” (only the smaller number of men are saved). And a few years ago, when the Jesuit P. Castelein (“Le rigorisme, le nombre des élus et la doctrine du salut”, 2nd ed., Brussels, 1899) impugned this theory with weighty arguments, he was sharply opposed by the Redemptorist P. Godts (“De paucitate salvandorum quid docuerunt sancti”, 3rd ed., Brussels, 1899). That the number of the elect cannot be so very small is evident from the Apocalypse (vii, 9). When one hears the rigorists, one is tempted to repeat Dieringer’s bitter remark: “Can it be that the Church actually exists in order to people hell?” The truth is that neither the one nor the other can be proved from Scripture or Tradition (cf. Heinrich-Gutberlet, “Dogmat. Theologie”, Mainz, 1897, VIII, 363 sq.). But supplementing these two sources by arguments drawn from reason we may safely defend as probable the opinion that the majority of Christians, especially of Catholics, will be saved. If we add to this relative number the overwhelming majority of non-Christians (Jews, Mahommedans, heathens), then Gener (“Theol. dogmat. scholast.”, Rome, 1767, II, 242 sq.) is probably right when he assumes the salvation of half of the human race, lest “it should be said to the shame and offence of the Divine majesty and clemency that the [future] Kingdom of Satan is larger than the Kingdom of Christ” (cf. W. Schneider, “Das andere Leben”, 9th ed., Paderborn, 1908, 476 sq.).
The church does not profess to know the number of elect. If it said most were saved, laxity may result (well, not that it already has). If they were to say that most were damned, it is possible that many would be led to despair.
The important thing is that we know how to attain salvation: Avoid mortal sin (which entails trying one’s hardest to not commit deliberate venial sin as well, since repeated venial sins incline one to mortal sin). If you do fall into such a misfortune as committing a mortal sin, try to make a perfect act of contrition and make a valid confession with a firm purpose of amendment.