Yes, you are correct. That statement is found in Ellen White's writings.
She wrote: **"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."** (
ChS 41.1)
This was a serious appeal for church members to examine their own spiritual condition and not rely on simply being a member of the church.
Interestingly, there is another quote that uses the "one" and "twenty" ratio, but in a very different context. It is about the need for more ministers:
**"The number of workers in the ministry is not to be lessened, but greatly increased. Where there is now one minister in the field, twenty are to be added; and if the Spirit of God controls them, these twenty will so present the truth that twenty more will be added."** (
GW 65.5,
6T 414.3)