Link velocity is the rate at which a site gains new backlinks, usually measured in referring domains per month.
Natural growth is uneven: a steady trickle with occasional spikes when something gets attention. What looks suspicious is a sudden jump of many low-quality links from unrelated sites, the pattern you get when links are bought in bulk.
Steady, relevant outreach produces a healthy velocity on its own. There's no ideal number to aim for.
Example
Margin gaining 6 to 10 new referring domains a month from relevant blogs is a healthy pattern; 300 in one week from directories is not.
Putting it into practice
Where to use it: Outreach volume planner.