I do not think there is any limit. You can collect as many as you want in a day. The impotant point is don't get banned. Collecting negative ratings without getting banned is an art. Wish you all the Best!
Not talking about collecting - but assigning. I don't think I have been given a negative rating, for a while now.
I do not think you have the power to give positive or negative ratings. Only PDF Think Tank members have that power. So stop worrying about things you do not control.
You can collect as many as you want in a day.
The impotant point is don't get banned.
Collecting negative ratings without getting banned is an art.
Wish you all the Best!