I think the best source for a brief understanding on the Lutheran view of the sacraments is Melanchthon’s writing in the Apology of the Augsburg Confession.
If we call Sacraments rites which have the command of God, and to which the promise of grace has been added, it is easy to decide what are properly Sacraments. For rites instituted by men will not in this way be Sacraments properly so called. For it does not belong to human authority to promise grace. Therefore signs instituted without God’s command are not sure signs of grace, even though they perhaps instruct the rude [children or the uncultivated], or admonish as to something [as a painted cross]. 4] Therefore Baptism, the Lord’s Supper, and Absolution, which is the Sacrament of Repentance, are truly Sacraments. For these rites have God’s command and the promise of grace, which is peculiar to the New Testament. For when we are baptized, when we eat the Lord’s body, when we are absolved, our hearts must be firmly assured that God truly forgives us 5] for Christ’s sake. And God, at the same time, by the Word and by the rite, moves hearts to believe and conceive faith, just as Paul says, Rom. 10:17: Faith cometh by hearing. But just as the Word enters the ear in order to strike our heart, so the rite itself strikes the eye, in order to move the heart. The effect of the Word and of the rite is the same, as it has been well said by Augustine that a Sacrament is a visible word, because the rite is received by the eyes, and is, as it were, a picture of the Word, signifying the same thing as the Word. Therefore the effect of both is the same.
The confessions make a distinction between the three instituted by Christ with the promise of grace - Baptism, Absolution, and Eucharist, and the four other very important rites of the Church.
To be sure, all four, marriage, Confirmation, Unction, and ordination are maintained within the Lutheran tradition of the Church. I, myself, was confirmed with the laying on of hands after catechetical training, married in the Church, and witnessed the ordination of a pastor, with the laying on of hands.
Further, Melanchthon makes this comment on the numbering:
Lastly, if among the Sacraments all things ought to be numbered which have God’s command, and to which promises have been added, why do we not add prayer, which most truly can be called a sacrament? For it has both God’s command and very many promises; and if placed among the Sacraments, as though in a more eminent place, it would invite men to pray. 17] Alms could also be reckoned here, and likewise afflictions, which are, even themselves signs, to which God has added promises. But let us omit these things. For no prudent man will strive greatly concerning the number or the term, if only those objects still be retained which have God’s command and promises.
I continue to be believe that, for Lutherans at least, the numbering is not a significant issue, one that for us is Church dividing. I know that I have received grace through my confirmation, and my marriage, and I am regularly blessed by the grace that comes through the hands of the ministerial priesthood.