How many sub text can you handle?

wanted to know, if two people are talking at the same time could you keep up with the subs, im able to keep up with 3 sub, two people taking and references. I rarely have to pause the anime to read but their are some special occasions that i have to... so my question is; How many subs can you handle?

I can handle about 2 or 3 layers so long as they are not purposely talking super fast. About the only Anime I couldn't do this with was a certain episode of Excel Saga where as part of the oke Excel was talking extrememly fast so the subs rushed along before I could read every part of it. So I think it's the speed more than how many different lines there are of subs, for me at least.

I can honestly say I can handle most of it since my understanding of basic Japanese helps but as for layers I guess just 2 tops if two persons are talking