You can give as many baby boost shots as you want (and can afford). What "diminished effect" means is that, after the first shot, each additional shot provides less of a boost than the one before it, but it still provides some amount of boost.

When it comes to using Baby Boost, you really only have two choices: guesswork or paying for a doctor's consultation. The doctor's consultation will let you know for sure where Baby Boost might be useful. Guesswork can be expensive, but may be effective anyway. Regardless of which option you choose, multiples are still largely random. You could spend all of your money on Baby Boost and still not get twins or triplets.

The description says, "Note that repeated applications have a diminished effect." That means that each additional shot will be less effective than previous ones, but it does not have a negative effect on ability to have babies.