Each of my big looms has eight shafts, also called harnesses. Each harness has lots of heddles. How many heddles you need on each harness is determined by the width of your weaving project and the structure of the pattern.
I'm weaving some wonderful Huck Lace kitchen towels on my Schacht loom. This pattern only requires four harnesses. If these towels have 520 warp threads, then I need 520 heddles total across the number of harnesses I'm going to use. It isn't hard to figure out. I looked at the pattern and figured out that I will need 102 heddles on harnesses 3 and 4. I will need 153 heddles on harnesses 1 and 2. OK, then. I just need to count the heddles.
I need to point out that I rarely if ever count my heddles. I start off with lots and lots of heddles on each harness and I'm very happy to have extras off to the side. Since I rarely weave projects as wide as possible there isn't much of an issue. I use a plastic paperclip to keep the extras out of the way and I weave on. But these Huck Lace towels only use 4 shafts rather than my normal 8. That means with a similar number of warp threads, each shaft will need about twice as many heddles as I usually need. Hmmmmm. Yes. I need to count the heddles.
That is not to say that I haven't gotten burned. When I was threading my big Louet loom for linen bath towels that are almost as wide as the loom can handle, I ran short of heddles on several of the harnesses. Most looms are designed so that it's pretty easy to add heddles if the loom is empty. You can remove each harness and set it on your desk or across a chair - whatever will make it easy to get to the ends - and add the heddles. If the harness is partially threaded, however, your options are limited. You can still add heddles but it is a royal pain.
So before I started to warp the looms I counted heddles. On the Schacht loom, the one where I was only using four shafts instead of all eight, I ran out of heddles right at the end. I needed 8 more heddles to finish up the warp. Fortunately, because I didn't use all the shafts in the design, I could use them for those 8 threads. It wasn't pretty and any weaver would have looked at that and laughed but it works. And I'm weaving away.
On the Gilmore loom I was using seven of the eight shafts. I carefully counted the heddles so I knew I had enough. And I ran out of heddles. Really? Well, crap. This time I didn't have extra shafts I could use so I had to painstakingly add the extra heddles to the end of each shaft while the shafts stayed on the loom.
So what are my takeaways from this? Obviously, don't count heddles. I seemed to make less errors when I didn't count them. Or maybe don't count heddles until you are capable of counting properly to 100 or 200 or whatever is required since that seemed to be my problem. Or maybe add another 100 or so heddles to each harness the next time the loom is empty and then don't ever count them again. The last possible option is to learn how to calculate the number of heddles you need properly and then count the damned heddles properly. Technically the best option but easily the most boring.