On modern computer hardware, as of 2013, the practical maximum number of points that you can collect with Logger Pro or Logger Lite is around a million. The practical maximum number of interfaces you can use is around 8 for Go! or LabQuest interfaces, but around 5 if the interfaces are LabPros. Slower/older computers will have more problems at those numbers whereas newer computers will probably be able to do more. (Trying to collect a million points from 8 interfaces at the same time will cause Logger Pro to crash.)
Logger Pro has a technical absolute limit of 500,000,000 cells of data, (32 bit programs are limited to 4 Gigs of memory including overhead, and each data cell is stored as 8 bytes of memory each,) but the points are stored multiple times in multiple formats, (the data table is text but the graph needs numbers,) so in normal operating Logger Pro will give a memory error long before 5E8 cells. We store things multiple times for many reasons, such as to make the data table faster to scroll. Note that on most new computers adding more RAM to the machine won't significantly change anything here. However, long before you reach the memory limits, things like redrawing the graph and checking the graph range for autoscaling, (let alone doing a curve fit,) will just be very slow so the software will become unusable. Deleting the data table and unnecessary graphs from your experiment files helps a little with this.
While it is easy to instruct Logger Pro to collect more than a million points, in most case it isn't useful to do so. For instance, you can tell Logger Pro to collect temperature data at 100 times a second for several hours, but is your object (and the sensor itself,) actually changing temperature that fast? The weather stations at most airports only updates once an hour: it isn't a question of technology, outdoor air temperatures just don't normally change very fast so weather data for 100 years is still less than a million points.
The USB bus is technically limited to 127 devices, (note this will take a lot of hubs and those count against the device total.) Latency on the USB chain starts to increase if there are a lot of things connected and talking at once, and if Logger Pro doesn't receive replies from interfaces in a timely manor it assumes the interface isn't there anymore and disconnects from it. That isn't a problem in normal situations where you have an interface or two on the USB port, but with enough devices all talking at once, even just updating the meters is problematic and the interfaces can start to disappear from Logger Pro. Using different root hubs on the computer tends to make this better.
The reasons for these limits are design tradeoffs: for most experiments done in an educational environment these limits aren't a problem and features like graphs that update automatically and live readouts are useful. For more details see our product usage section here: http://www.vernier.com/legal