How to dynamically allocate memory for a structure with an unknown number of elements?

I need to open a file and read in X number of analog input channels that my program will log.  I have gotten by in the past with the following allocation:

static	struct	DAQChannel 	*DAQAIChannels[100];  

DAQAIChannels[numAIChannels] = (struct DAQChannel*) malloc(sizeof(struct DAQChannel)); 

The trouble with this is that, if I don't allocate memory for all 100 channels, I get an error when I try to free all of the memory like this because some of the memory hasn't been allocated:

free(DAQAIChannels);

How should I use pointers such that I can allocate and free memory dynamically for an unknown number of AI channels?  I should be a little more clear that, as I read line by line from the file, I create a new index in the structure array so the number of channels is not known until the last AI channel line is read.

Thank you!