More is better, it's that simple.
To give you something to aim for:
Minions spawn at 1min30. They take roughly 30sec to get to lane, 30sec more to clear the wave.
So at 2:30 you should be clearing the first wave. At 10min you should have cleared 16 waves (5 of which had a cannon) for a total of 101 minions.
Each 10min thereafter is another 20 waves (6.67 of which have a cannon) for a total of 126.67 minions.
So 100% CS should result in the following amounts at 10min intervals:
10min - 101
20min - 228
30min - 355
40min - 481
Of course, no one (not even the pros) hits 100% CS. Even if you're a god of last hitting (and no word of a lie, I've had games where I've not missed a last hit for the first 10 minutes) you're still missing CS due to basing for items, roaming, securing dragon, securing kills, etc...
You can also theoretically get 'over' 100% CS - as jungle creeps count towards this as well. So if you get ahead and can fast-clear your wave then go do raptors / krugs / gromp / wolves before the next wave comes you'll develop an xp and gold lead.
Anyways, on to the point:
Aim for as high a % of CS as possible. Anything under 50% is unacceptable outside of low bronze (unless you're making a lot happen elsewhere).
I'd suggest to start working for 60%. Then 70%, then 80%. Once you get to 80% of the above numbers, you're probably doing a very fine job CS'ing.
To help the metric feel free to count the following:
A kill = 15 cs
An assist = 10 cs (it's not actually worth that much, but you gave an ally a kill).
participated in dragon = 6-10 cs
participated in baron = 20cs
On top of that, often times relative CS is more important than absolute CS. If you're Caitlyn bot lane versus Vayne, which situation would you prefer?
20min - Caitlyn at 220cs, Vayne at 180cs
vs
20min - Caitlyn at 175cs, Vayne at 90cs
Clearly the latter case is really shutting Vayne down and is more beneficial to your team. It's worth trading heavily, requiring lots of bases, making them miss last hits by pushing in to tower, etc... as long as you're getting more than your lane counterpart. This is especially true if your counterpart scales harder than you (it doesn't matter if you're 6 kills ahead if the enemy hypercarry is 4 kills ahead of any of your allies). Getting off-topic here though, so I'll just stop typing.
