Whats everyone doing?
Ten round would be enough to give you a good idea of average velocity and SD.
Just guessing here, but I suspect - like me - you can't wait for the Labradar. . . If your doing OCW or ladder tests, shooting over a chronic that isn't changing your POI like my Magneto Speed (I still have two chronographs), allows you to accomplish the visual while at the same time figuring out *if* the node is even in the speed range you need and *if* your proposed load is going to have a low enough ES to be useful past several hundred yards. . . .
This is really a statistics question.
In the field of six sigma, there are varying opinions of how large a sample population must be for you to run a good statistical analysis. Usually that number is between 25 and 35 - and that's IF you can truly consider your sample population "random".
Loading a batch of ammo once and checking it's speed would not be considered a truly "random" sampling. Better would be to chrono 2 or 3 rounds from each batch of ammo you load, and after a dozen batches have been chrono'd, calculate the mean (average) and standard deviation (SD) for that entire 25-35 round sample population.
SD has a very significant purpose, and lots of really smart people have devoted their careers to how to properly implement it.
In short, assuming you've got a GOOD measure of standard deviation...
Your mean value +/- 1 standard deviation will give you a spread that will represent ~69% of ANY SAMPLE EVER. That means any random sample you take later has a 69% chance of falling into this spread.
Your mean value +/- 2 standard deviations will give you a spread that will include ~99.4%
Mean value +/- 3 SDs will include 99.9997%
...which is why "six sigma" (+/- 3 SD's) is the gold standard of goodness.
So ideally, you'd fire 30 truly random rounds over the chrono, find your mean and your SD, multiply your SD by 3, then add and subtract 3 SDs from your mean to give you a true hi/low limit of what you could expect 99.9997% of the time.
How much dilution of the ideal can you get away with and be ok? I'm pretty happy buzzing 5 or 6 over the chrono before a match and using whatever average it calculates. Sad fact is our rifle barrels are ever changing. I shoot a 6mm that seems to slow down ~10fps every 100 rounds once it's got 500-600 on it.