Answers to your questions:
Online documentation of the IQFeed API can be read at http://www.iqfeed.net/dev/index.cfm?login=login.
You will need to login with your website username and password, which you received when you subscribed to the API product. If you don't know it, that URL has a "send me my password" link. You just need to enter your email address. If you still can't retrieve it, contact IQFeed support.
The most basic way to build historical bars is a history command, as per https://www.iqfeed.net/dev/api/docs//HistoricalviaTCPIP.cfm.
What you probably want is an HIT command. This allows you to specify a symbol, interval, start/end date/time, and get a list of all intervals in that range. This will return Timestamp,High,Low,Open,Close,Volume,Period Volume.
If you want to collect historical bars and gather new bars as they come in, the BW command works similarly.
Yes, tick data generally goes back 180 days. Minute and daily data goes back further.
There is no limit to concurrent connections, provided they all come from the same computer. To have concurrent connections from different machines would require multiple IQFeed login IDs.
There can be 15 history requests active at one time. "Active" means the request has been sent and the data has not been completely returned yet. If you are requesting large amounts of history for a large number of symbols, you might have to space your requests out a little. But most requests process so quickly this isn't an obstacle.
If you are doing any kind of live "watching" of a symbol, you may only watch up to 500 symbols at a time, unless you pay an additional fee to view more.
There are no per-time period maximums on the total number of historical requests you make.
DTN IQFeed Implementation Support Specialist