I guess some more information on what you are trying to do would be helpful. But I'll start here: if using stock big block heads, there is only one accessory hole, if you want to call it that. The heads are cast such that the mounting boss for the alternator bracket is on the passenger side. That is, the same head, used on the driver side, will have the mounting boss facing the firewall. There is no "formal" or design, mounting boss cast into the other end. I am not saying there isn't enough material to drill and tap, its just not a factory mounting location, so its not machined flat and drilled. I think on all of the big blocks, any accessories such as power steering and air conditioning, the additional brackets were mounted to holes off the water pump housing. So not additional holes in the heads. I've never had an AC or PS car, so not 100% sure. Back in the 60s and 70s that was about it for accessories.

Now if using aftermarket heads, there may be an additional option. I think heads from Edelbrock and Promaxx have thick end castings. You may be able to drill and tap into that. But, you run the risk of breaking into coolant passages. Not the end of the world, you just need to seal up the threads. Though some of the aftermarket heads are based off the stock cast iron heads which would just give you the one mounting boss.

If you look at aftermarket serpentine belt kits, procharger kits, etc. they all mount to plates that are mounted to the alternator and water pump locations.

Let us know what you are trying to do and maybe we can provide more detail.