Will only one in ten Adventists be saved?

No, Mrs. White did not make such a statement. In the CD-ROM database, there is only one reference to “not one in ten,” and that is addressing the question of how many ministers are suitably informed and competent to handle health questions. Mrs. White’s more common expression was “not one in twenty.” The closest we can come to what you asked for is this statement:

It is a solemn statement that I make to the church, that not one in twenty whose names are registered upon the church books are prepared to close their earthly history, and would be as verily without God and without hope in the world as the common sinner. They are professedly serving God, but they are more earnestly serving mammon. This half-and-half work is a constant denying of Christ, rather than a confessing of Christ. So many have brought into the church their own unsubdued spirit, unrefined; their spiritual taste is perverted by their own immoral, debasing corruptions, symbolizing the world in spirit, in heart, in purpose, confirming themselves in lustful practices, and are full of deception through and through in their professed Christian life. Living as sinners, claiming to be Christians! Those who claim to be Christians and will confess Christ should come out from among them and touch not the unclean thing, and be separate (Christian Service, 41).

Though this is certainly a solemn statement, you can see that it is not a prediction of what will happen at the end but is a statement of conditions current at the time of the writing. The same may be said of her other statements of “not one in twenty” or “not one in a hundred.” The outcome at the end will depend on the response of people to appeals such as those Mrs. White made and to the final events as they unfold.

We shouldn’t take these statements literally, because never once does Ellen White write “not one in eight” or “not one in thirteen.” She used the statements figuratively, to show the seriousness of the point she was making.