Binance API questions

If this is the wrong place, please let me know where to ask. I haven't been able to find the information on Binance's website.

All questions relate to binance.us and using the API.

1. Is there a time limitation on how many times I can buy and/or sell USDT?
   
2. For example, can I purchase and/or sell every .1 seconds?
3. Is there a fee for buying or selling USDT?
4. Can I own more than one bag of a coin or token at once or will it always default to a single bag when I make a purchase and already own a bag of that coin or token?

Thanks for any info!