The point of the glycerol used for storage of bacteria is preventing freezing damage, such as ice crystal formation. At lower glycerol concentrations like 10-20% the frozen stock will be fairly solid at -80C, while at higher concentrations (30-50%) it may remain partially liquid. Either is fine but over time (years) the cells will settle down at bottom if still liquid. So, anything over 10% should be ok. Normally, 20%, 25%, 40%, 50% are often used.
Souce: NovoPro 2018-02-07