The best and easiest way is to use a single larger VPS to handle the different traffic apps. If uptime and stability are more matter in your case then you can run one app per VPS in critical services. Also, you can do a hybrid approach. The hybrid approach contains the separation of resource-heavy apps and low-traffic apps ones together. It can also work well.
If you consider the cost then a single VPS is easier to manage and allows flexible resource allocation. But, it creates an issue like one application could impact others. Multiple smaller VPS instances provide better isolation and redundancy to one service that stays online if another fails.
I prefer to use a single 4GB vps to a single app mostly. Still, if you want to use one vps for all apps then you have to prefer at least 8GB vps for sure.