What is the grammatically best way to put this?

"The number of people killed by the virus will reach one million."
or
"The number of people who have died because of the virus will reach one million." 

AmulyaP2020-09-29T04:46:49Z

Both sentences are correct, precisely formed keeping grammar rules intact.
The second sentence has two clauses.
Now 1.. Principal Clause  .. The number of people will reach one million.
No 2. Subordinate Adjective Clause...Who have died because of the virus

By the way, I would prefer the first sentence for it is brief.

GuantanamoGeorge2020-09-29T03:14:21Z

Tenses have to agree. The present perfect "have died" does not agree with the future "will reach."  "The number of people who have died because of the virus has reached one million" is grammatically correct but changes the meaning.
The first sentence is correct because "killed" is used as an adjective, not the past tense. 

Anonymous2020-09-29T00:07:27Z

Both are grammatically correct, and both sound equally normal or idiomatic.