As above, depends on the activity, but also your proxies too.
Private proxies which are high speed will obviously perform much better than shared proxies.
I tend to stick to a ratio of 10 threads per private proxy. I use this ratio when registering/posting or anything similar.
If I am going to be scraping a massive list, I tend to put it 3:1 with at least 10 private proxies in use, this can last a long time before getting blocked. If small lists or fewer keywords/terms, 10:1 is fine too.