"come up with a number" is correct and is commonly used in written English. Here is an example sentence: I can't decide what I should charge for my service, so I'm going to have to come up with a number.
Let's come up with a number".
"I'm not sure how you come up with a number".
They have come up with a number of strategic responses.
It didn't take long to come up with a number.
Now, I've come up with a number of 35percentt.
Experts have come up with a number of ideas to "save" the Dead Sea.
"To come up with a number doesn't seem to make sense," he said.
The only player who has ever come up with a number is Tiger – 19.
Since then, I've come up with a number of reasons for why it is so dull.
"We came up with a number, but we did not seek a number," Mr. Reyes explained in a recent telephone interview.
The research group came up with a number of topics.
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Ha Thuy Vy
MA of Applied Linguistic, Maquarie University, Australia