What does "Numbers err in this" mean?

You cannot separate the two phrases, because they are linked in the meaning.

But, of the two, less dang'rous is th' Offence,
To tire our Patience, than mis-lead our Sense:
Some few in that, but Numbers err in this,
Ten Censure wrong for one who Writes amiss

It is less dangerous to tire our patience than it is to mislead our understanding. Some few mislead our sense (our understanding), but habitual criticism is misleading, because ten critics will censure one person who writes wrongly (amiss), who misleads our sense (our understanding).

The use of the word sense has changed dramatically since the 19th century. Today, we use it in such contexts to mean an idea, often a vague idea (I get the sense of what you're saying, but you'll need to explain it in more detail). But in the 19th century, sense meant understanding. No one would have used it to mean vaguely understanding, rather they would use it to mean someone who understands perfectly well. We also now use the words sensitive and sensitivity, while in the 18th and 19th centuries, English speakers would have used the word sensibility to mean the same thing as we mean by sensitivity. So, Jane Austen entitles her first published novel Sense and Sensibility, meaning true understanding and a false sensitivity which actually results from a lack of understanding.

Thanks.

So "Numbers" refers to the number of critics?

Yes, that's how i read it.

I read it as "looking at the numbers does not tell an accurate story" since "tire our patience" is not equivalent to "mislead our understanding" so that if we are more often bored, it doesn't exonerate those who attempt to mislead.