You can use a total of six tags. That includes if you # something in the body as well as those you list in the little tag field.
I try to use tags that are already known topics but maybe not super popular. You want people to find your writing so really obscure tags won't help, but you also don't want to be lost in something as crowded as say, "Blog."
Thanks! I didn't know about the ones in the body counting. I suppose when you got all kinds of #yonihashtags which must become known to humanity then you have to sacrifice some of the other tags to the universe and trust that #ifyoubakeyonipietheywillcome