Hi Timothy,
This depends on how much memory is on the 6561 card, for instance if you have the 2 MB memory option this will allow you to generate 2 MSamples and Acquire 2 MSamples. The size of the sample i.e. 8 bit, 10 bit etc. really doesn't matter as the memory quoted is per channel so 16 x 2 MB. Also note that there is separate memory for generation and acquisition so 16 x 2 MB x 2 = 64MB memory total.
Hope this helps,
Nick