Pepe,
One tree will do it!
You don't need more that one tree to produce fruits. Cross pollination may increase production, but it is not necessary! I have a couple friends here in the area growing them and they have only one tree, each.
My tree was knocked down several times and I no longer grow one. My tree produced several fruits by itself.
Non-dwarf cashews may become huge trees. Get the facts! Some varieties are grown just for the nut (actual fruit). Others are grown for the pseudo-fruits. Some varieties, the pseudo-fruits are absolutely delicious and some others are pretty nasty! Good luck!