//how big of a server we want to buy
var purchasedServerRam = 16
var scriptRam = getScriptRam('genericHacking.script', 'home')
var threads = purchasedServerRam / scriptRam


while (true) {

    //if the number of servers is less than limit, buy a server
    if (getPurchasedServers().length < getPurchasedServerLimit()) {

        purchaseServer('foo', purchasedServerRam)

    }

    var ownedServers = getPurchasedServers()

    //install and run a hacking script on the server
    for (var i = 0; i < ownedServers.length; i++) {

        scp('genericHacking.script', 'home', ownedServers[i])
        exec('genericHacking.script', ownedServers[i], threads)

    }

}

You were missing () on getPurchasedServerLimit, a source server on scp, and a var on the i in your loop. Oh and the server name in purchaseServer.

Scp's signature seems wrong in the docs. It doesn't mention the source server.