If your account is below 25,000, you can only make 3 round trip trades per week. If you enter into your fourth trade - you will be flagged as a pattern day trader and your position will be stuck for up to 90 days. This is at least my understanding. Google "Pattern Day Trader"

is that just per broker, so if 2 brokers=6 trades per week

Per Broker according to Tim. http://www.timothysykes.com/2015/04/why-i-hate-the-pattern-day-trader-rule-and-what-you-should-do-about-it/