Totally not necessary, i have tone and tons. The last 2 packs i received i did not even bother punching the tokens out of their cardboard. I couldnt put a number on how many you need as that would really depend on your play-style.
Technically speaking though, if you played 1x starting quest, 3x act one rumors, 3 act one quests, 1 interlude, 3 act two quests, 3 act two rumors and 1 finale; then you could technically get a maximum possible of 90 threat tokens over the coarse of the campaign.
Though 2 threat tokens you would get from winning the finale would be pointless, so its more like 88 usable threat tokens.
Edit: Actually only 2 rumours are possible at the moment, because the current rumours cards do not let you play any other rumours from that same expansion. This will change shortly though with the release of the heroes and monsters pack and other mini expansions coming in the future. So currently the maximum amount of tokens you will ever theoretically need is 76.
Edited by BentoSan, 06 March 2014 - 11:09 PM.