The compound shown is treated with one equivalent of acid. Protonation occurs at which nitrogen, and how many pi electrons does the ring of the resulting cation contain?

The compound shown is imidazole. The N-H nitrogen lone pair is part of the aromatic sextet, whereas the pyridine-type nitrogen has its lone pair in the ring plane, so protonation occurs there. The ring then has one C=C (2 pi electrons) and two N-H lone pairs (4 pi electrons), giving 6 pi electrons, so the imidazolium ion remains aromatic.