Testing Trades that won't fill

I’m on the trial and testing the platform. I’m not giving any regard to performance or anything of that, but rather stress testing the capabilities.

Yesterday, I found that I couldn’t place a 21st order. It just wouldn’t go through.

Today, I seemed to be limited to even less, maybe 5.

Now, in general, I don’t ever take that many trades, but I want to understand what the limitation is about just for my understanding.