For my app, how many threads would be optimal?
You will probably find your application is bandwidth limited not CPU or I/O limited.
As such, add as many as you like until performance begins to degrade.
You may come up against other limits depending on your network setup.  Like if you're behind an ADSL router, there will be a limit on the number of concurrent NAT sessions, which may impact making too many HTTP requests at once.  Make too many and your provider may treat you as being infected by a virus or the like.
There's also the issue of how many requests the server you're crawling can handle and how much of a load you want to put on it.
I wrote a crawler once that used just one thread.  It took about a day to process all the information I wanted at about one page every two seconds.  I could've done it faster but I figured this was less  of a burden for the server.
So really theres no hard and fast answer.  Assuming a 1-5 megabit connection I'd say you could easily have up to 20-30 threads without any problems.
I have a simple Python web crawler. It uses SQLite to store its output and also to keep a queue. I want to make the crawler multi-threaded so that it can crawl several pages at a time. I figured i would make a thread and just run several instances of the class at once, so they all run concurrently. But the question is, how many should i run at once? should i stick to two? can i go higher? what would be a reasonable limit for a number of threads? Keep in mind that each thread goes out to a web page, downloads the html, runs a few regex searches through it, stores the info it finds in a SQLite db, and then pops the next url off the queue.