Most big VC firms ($600-$2b+ in a fund) want to own 15%-20%+ of a start-up, ideally even 30% in a Series A investment.
Smaller VC funds ($100m or smaller fund) would like to own 10% or more after a Seed or Pre-Seed Investment, but typically model around 7% average ownership.
VC funds may be more flexible with ownership percentages for non-core investments, but they won't be as engaged.
In late-stage deals, it's more about absolute $$$ invested at a reasonable price than % ownership, but in Seed, A, and B deals, ownership % is important.