  In the past the analyses of inelastic p+p collision for example in terms of
the hadron gas model have assumed that always both protons participate in the
interaction. In this article we show that (at sqrt(s) = 17.3 GeV ) on average
only 1,89 protons are interacting. Measurements of the mean multiplicities of
protons, neutrons, charged pions, charged kaons and {\Lambda} hyperons allow to
compute the number of initial state nucleons using conservation of baryon
number, strangeness, and isospin. We further argue that once the number of
initial state nucleons is given and the mean multiplicities of protons and
charged pions are known in p+p interactions at center-of-mass energies below a
few tens of GeV, the yields of net neutrons, kaons, and hyperons can be
estimated with a precision of a few percent using again baryon and isospin
conservation.
