Out of the quesiton depend on the league. If the league allow unlimited then that is what it is. And if man paying money to sweat, they eh sitting on the bench to keep tradition alive.
Now for the Real, big goal, FIFA sanctioned game - 3 subs is plenty.
If you are allowed unlimited subs, then the benefits depend on the style, players etc. If you have more subs, or more fit men than the opponent, then you could play run and gun, and then relax when they bun and cyah chase the game anymore. Run hard, sub quick, and send in the nex man to run hard. Of course, there's nothing pretty about run and gun, but it is effective.
I sure nobody want to see NFL style subs with a defensive team and a free kick team and a corner team and nuff coaches on the sidelin making sure you prepared for the next corner kick.