TL;RD: yes, you can run more than one proxy in the same machine. The number of proxies depends on your cpu/bandwith, but having 2 or 3 should be fine.
The standalone proxy has a -capacity flag, that if is not set (or set to 0) it will accept as much clients as needed. So you should not need to run more than one proxy in a machine. But each proxy tells the broker how many clients are currently connected and the broker will assign new clients to the proxies with less clients connected to them. So in theory you will get more clients assigned to you by running more than one proxy.
Would be nice to improve the broker assigning algorithm to use some better heuristics than how many clients are connected to the proxy, as not every proxy has the same cpu/bandwitch available. But is not trivial to improve.
There is also to take into account that the usage of the snowflake network is not always very high, so your proxy might be idle just because there is no so much usage on the network right now.
So yes, if you see that your cpu and bandwith are idle is not a bad idea to add more proxies. But don’t run too many as you might get surprised when the usage spikes.