Within Avalanche there is no set limit to the size of the IP address range.
When Avalanche is getting ready to run a test, memory for each IP address in the range is set aside from the memory pool (which varies in size depending upon the equipment in use). So the larger the IP address range, the less memory that is available for the remainder of the test. In the end, if the IP address range is so big that it does not leave enough memory for the rest of the configuration, the test will not run or may abort part way through.
As a recommendation, Spirent suggests keeping the IP address range as small as possible ( a couple of hundred addresses is ideal). While this may not provide a unique IP address for every connection, Avalanche will use a unique port number for each connection. If a unique IP is an absolute must, in addition to keeping the IP range as small as possible, it is recommended that you turn off as many features on the Run>Configure tab as possible as well as reducing the logging detail and lengthening the sample timer.