"greater in number" is correct and can be used in written English.
You can use it when you want to compare two numbers, one of which is larger than the other. For example, "There were more men in the room - 10 in total, which was greater in number than the 5 women present."
And are greater in number than we admit.
But the mistakes are much greater in number and effect than the harm caused by bad luck.
He explained that Australia's forces engaged in Afghanistan were greater in number than those of many NATO members.
The advertising world is still largely controlled by white men (with white women making greater strides in increased presence than black men and women), and the exceptions are still ultimately outliers, if somewhat greater in number.
We have no fewer wars than we once did — they are arguably greater in number, in fact — but there has been a sort of brutal efficiency in the march from the Second World War to Vietnam to Iraq.
After half a dozen blows we were pulled apart by assorted peacemakers, greater in number as the years have passed, a bit like the IRA men who later claimed to have been in the Dublin post office during the Easter Rising.
Our lawyers and doctors are as great in number as the populations of some Arab countries.
These letters, discovered after her death, are highly personal and, being so great in number, they chronicle Larkin's feelings more intimately than anything we have read before.
When desert locusts (Schistocerca gregaria) become too great in number, they transform from solitary insects into swarming crop raiders.
This is cause for great celebration within the camp of Silva/Allon fans and they are great in number.
Extension responses were initially by far the greatest in number and duration.
