How often can I get SC vit. B12 inj. If I don't have a deficiency but just want an extra boost of the vitamin?

No good interval . Before you start such a program you need discuss this with your physician and if your physician agrees you should have a baseline blood value of your vitamin B 12 before you start . There should be little downside of who doing this.
Shouldn't at all. Vitamins do very little good if there's no deficiency so you shouldn't waste your money. Spend it on healthy food and activity you enjoy.