The phrase "a decisive number" is correct and usable in written English.
It can be used when referring to a number that has a significant impact or influence on a situation or outcome.
Example: "The study revealed a decisive number of participants favored the new policy, leading to its implementation."
Alternatives: "a critical number" or "a determining number".
How did the Democrats lose a decisive number of Obama voters to someone like Donald Trump?
And some — perhaps a decisive number — will say, It's about time.
In so doing, they enabled a decisive number of marginal constituency seats to be delivered straight to Labour.
Southerners -- a decisive number of them -- believed the national Democratic Party did not share their values, and they did not trust the national party with their money.
The most conspicuous clichés to fall, of course, were the twin suppositions that a decisive number of white Americans wouldn't vote for a black presidential candidate — and that they were lying to pollsters about their rampant racism.
By contrast, Trump and company are counting on a decisive number of voters to make the old connections of government action with bureaucracy, bureaucracy with central planning, central planning with inefficiency, inefficiency with decline and decline with repression.
They hold seven seats on the committee, a potentially decisive number, since the panel has 36 Democrats and 23 Republicans.
As many as 40 Democrats, a potentially decisive number, have said they may oppose the bill without tighter restrictions on abortion.
Meanwhile, a small but decisive number of manuscripts of the Compositiones are completely independent from the Mappæ tradition.
They were among a small but potentially decisive number of first-time legislators.
Traditionally, governors have been able to leverage the governmental and political apparatus they control to deliver to their party's presidential aspirant a small but sometimes decisive number of extra votes.
