Graphene nanostructures are a promising platform for engineering electronic states with tailored magnetic and quantum properties. Synthesis strategies on metallic substrates have made it possible to manufacture atomically precise nanographenes with controlled size, shape and edge geometry. In these nanographenes, finite spin magnetic moment can arise as a result of many-body interactions in molecular orbitals with $π$-conjugated character and subject to strong spatial confinement, for example at the zig-zag edges. However, owing to the mixing of the molecular orbitals and metallic states from the catalysing substrate, most of their expected quantum phenomenology is severely hindered. The use of in-situ ultra-thin decoupling layers can impede nanographene-metal hybridization and facilitate the expression of predicted properties. Here we show that the edges of narrow chiral graphene nanoribbons (GNRs) over MgO monolayers on Ag(001) can host integer charge and spin-1/2 frontier states. The electron occupation varies with the GNR length, which alternates even or odd number of electrons, thus resulting correspondingly in a non-magnetic closed-shell state or an open-shell paramagnetic system. For the latter, we found the spectral fingerprint of a narrow Coulomb correlation gap. Charged states, up to 19 additional electrons, were identified by comparing mean-field Hubbard (MFH) simulations of the density of states with experimental maps of the discretized molecular orbitals acquired with a scanning tunnelling microscope (STM). In consideration of the length-dependent magnetic moment and the discrete nature of the electronic structure, we envisage that GNRs supported by thin insulating films can be used as tailor-made active elements in quantum sensing and quantum information processing.