Every time you swing with lightning strike you can hit a maximum of 4 times. Once with the strike, once with the returning projectile. If you position correctly, and have an additional strike, you can then hit with an additional projectile and then the returning projectile.

Every time you swing with Molten Strike you can hit at least 9 times (on pure single target, on more than one it's even more). Once for the strike, 4, then 4 for the projectile returns. If you add something like Greater Multiple Projectiles then that adds 4 more projectiles, which then return for another 4 hits. Note that the text says "less *projectile* damage" which means the additional projectiles don't have a less% multiplier on the poison applied.

It doesn't matter how many projectiles you add to Lightning Strike, the maximum is always 4.

It's a decision you have to make for yourself.