What/How many devices to put in one local ground net?
You could estimate the xx nH + xx mOhm per via and reduction in ESL with transient noise from Ic=CdV/dt and V=LdI/dt and give a noise budget for supply and ground ripple and estimate spacing to &lt; 5% wavelength, but we can only guess your application.  So you should observe best practices in similar applications and decide what your PWB shop suggests for microvias.

This site is consistent with my thinking. http://www.interfacebus.com/Design_Capacitors.html  

Keep decoupling cap close to each IC. 0.01uF per MSI IC for best immunity with direct connection to ground/Pwr plane for &lt;=5V Logic. It depends on how many gates are being switched synchronously.  SSI gate IC's can use fewer vias and decoupling caps but it depends on what noise margin specs you define. 

So no explicit Rules can serve all applications.