Actually, spirulina isn't the highest form of protein you can get. The highest form is protein from milk or eggs followed by meat protein. Spirulina is better than most plant forms of protein though because it's a complete protein. It's not as good as milk, egg or meat protein because it has reduced amounts of methionine, cysteine and lysine compared to them. It also costs about 30x more per gram of protein than those sources.
Everyone will tell you something different as to how much protein to get. My program starts people off at 70-90 g and says that getting at least 70 g a day is for life, not just during the weight loss phase. Some people say that RnYers should get 100g (in order to actually absorb 70 g).