How does the number of electrons increase in beta decay?
The two commenters have pretty much answered your question: after beta emission, which is a sufficiently energetic process to tear apart a molecule, the resulting fragments/ions would likely undergo a series of additional reactions with any nearby molecules to finally produce stable species. The emitted electron also would be captured by nearby molecules, leading to a cascade of reactions, also eventually producing stable species. In any case you do indeed end up with a nitrogen atom and an extra electron.

Example: if decay occurred in a bottle of liquid methane, you would get ammonium cation (isoelectronic with methane) and an extra electron loosely attached to methane - a methane radical anion. These could recombine to give neutral ammonia and a hydrogen atom, and then further reactions are possible.

I would also point out that carbon 14 has 6 protons and 8 (not 12) neutrons, but in any case your question remains valid.