Link velocity is speed of creating back links per day or week. Its being said that don't create too many backlinks in a day as that might be caught by google as blackhat seo. Just creat upto 10 backlinks everyday if you can
There is no hard and fast rule for creating back links.If you are getting links in natural way , so there is no limit for creating links and if you are getting links by partispating in different link schemes , than you should have to think for the limit of creating back links.
Link velocity is simply the rate of incoming links to a website. Excessive/bulk amount of backlinks gained at small period of interval is a risky thing and there might be chances for Google to lower the site ranking.