I'd say more than four.
Anacondas are tough, starting lasers on a Sidewinder are very weak, and even then, Sidewinders would have to take turns drawing the line of fire of the Anaconda while the others wait for their shields to recharge, flying around its weak angles and giving it all they've got. How many Sidewinders are needed for that will depend on how long they can evade the line of fire, and how fast the Anaconda can recharge its shields. And it would take a very, very long time.
I've fought NPC Anacondas in a starting Sidewinder before, and everytime I gave up once I realized that it took ages to bring down its shields, only to damage it slightly before they recharge.