BUT, if you like to save some time in upgrades, probably you will like to make a multi-tenant installation (single ojs for multiple journals).
Despite the time gained from maintaining the OJS, the opposite happens on the users’ side: the downtime for making upgrades is increased. In this sense, it is a point against several journals in the same installation of OJS, as the upgrade process can be time consuming.
I also advocate single-tenant, even if it makes me have to deal with hundreds of OJS / OPS / OMP installations today. 
It would be interesting alternatives to reduce this problem, such as: