Welcome to the forums, davidboy.
That's poetic licence for you.
I agree that grammatically it would be better as
'We've no fewer days to sing God's praise
Than when we first began.'
Student or Learner
I've always been hung up on this verse of Amazing Grace for two reasons:
When we've been there 10,000 years
Bright shining as the sun,
We've no less days to sing God's praise
Than when we've first begun.
1. It seems to me that the final line should technically utilize the past perfect and therefore read, "Than when we'd (we had) first begun." (implying that the start of the singing of God's praise begins later than now) or potentially, "Than when we first began."
2. Third line should be, "We've no fewer days..."
...although I'm not necessarily advocating the butchering an old hymn of the church, I think #1 at least maintains the pseudo-rhyme while correcting the grammar.