How many layers of clothing on Autumn in these US cities on September?

I suggest wearing layers then if you are too hot you can take something off or if you feel cold you can put something on. Like you need underwear then a t-shirt on, next trousers and socks, then a cardigan or sweatshirt or jumper and finally a light weight coat that if waterproof. You would take the coat off whilst indoors. In Florida it is likely to be warm and so cotton trousers would be good or it might be hot so cotton shorts would be good. A t-shirt there would be a great idea. There you might want sandals in the other place you should have shoes. The weather might be warm in Los Angeles or it could be cooling off because you are going in the autumn. Have a wonderful time in the USA.