I don't think you need any more than 2 linesman. The natural "human" way of calling offsides worked well because in some cases it is a judgement call. And when players were "about level" generally the benefit of the doubt would be given to the attacker, which worked quite well. Now with VAR it's a cluster, with armpits and toenails being used to chalk off goals. It's gone completely mad under VAR, with linesman letting play continue then putting the flag up after goals are scored to "check".
What I would advise in a world without VAR is in major competitions, they could put 2 additional assistants on each end line next to each goal. In case the referee misses something. Generally when the referee misses something or has his view blocked it's in the box near the goal because he's sprinting back and forth up and down the pitch. Even if something happens in the middle of the pitch, the assistant referee on the endline might be able to catch something he missed there too.
If you had an assistant referee stationed on each endline next to the goal, he would catch something like the infamous Henry handball, then notify the main referee. That simple procedure of putting another set of eyes on each endline would likely catch 99.9% of the missed calls that occurred pre-VAR. And he would have a better vantage point to determine who kicked a ball out for corner kicks / goal kicks. Endline assistants would work very well in limiting howlers and you wouldn't need any of this VAR nonsense.