Although Mystery111 has already given you a good answer, I thought it may be of benefit to you if I went into some further explanation-
As has been said, the reason a select amount of elements exists regardless of the potential combinations string theory allows is because of the Pauli exclusion principle. This is the principle that states that particles of like charges will repel each other, ie. protons will repel protons, electrons will repel electrons. The strong nuclear force allows atoms to exist despite the close proximity of protons within the nucleus, however, there comes a point where there are enough protons in the nucleus of an atom for the 'outer' protons to be far enough away from each other for the Pauli exclusion principle to come into play, thus the atom is unstable. So in the case of naturally occurring elements, 92 is the max number of protons that can successfully exist within the nucleus (Uranium).
As for strings in a vacuum (given that we are referring to a theoretical 'complete' vacuum), strings would still exist, despite the vacuum being devoid of matter, due to the presence of vacuum energy. As for the quantity, I have no clue.
I hope this has been helpful.