It depends on the algos you have on each and how resource intensive they are.

For our algos you can reasonably have 3-4 MT4 terminals without issues on FX VPS 1, but I can't say anything about other algos.

My general rule of thumb is that the idle CPU usage should stay generally below 15-20%, and the usage should spike higher during candle changes when our algos will be evaluating the market changes and still have enough to work.

The CPU usage just as you connect to RDP to see the VPS tends to be higher than when the VPS is running by itself so I let it “settle” for 5 or so minutes before I check.

The main thing is that you don't want execution to be delayed when needed so keep an eye in your notifications and trades, if they start getting delayed, even if the above looks fine then you may need to upgrade so that the algos have the resources available when it matters most (when placing trades)

I personally use FXVPS 2 and I have had at times more than 15 MT4 terminals running without issues.
that is having about 5-8 running algos and the rest running trade copy receivers so I can copy between tracking accounts and my other accounts. That can help give a scale as to what is sustainable with well-optimized algos and a simplified setup.

The type of algos and amount of them run at the same time will greatly influence what is reasonable or not at each VPS level as I have seen other traders with only 2 MT4 terminals in the same FX VPS 2 tier and they are getting significantly slow downs because an indicator or algo they use is taking too many resources.