Sensing 24v AC with 4N35?
From the datasheet.
The 4N35 isn't rated for an input of more than 50 mA, 0.05 A. So if that's not enough for your application (the output current of a 4N35 is approximately equal to the input current, and also isn't rated for more than 50 mA continuous), it's not a good part for this application anyway. But I don't think you'll need more than that; that's quite a lot of current for a signal.