GPIO0: pull low during boot for flash mode (connects to a push button). Other than that, usable as GPIO but beware of waht you connect to it.
GPIO1, 3: TX and RX (Serial). Also usable as GPIO if not using serial.
GPIO2: is pulled up during boot & internal LED (active LOW).
GPIO15: fixed external pull-down (for boot).
GPIO4, 5, 12-14, 16: nothing special.
So you have 11 usable GPIO including the Serial pins plus the analog input, but some you have to be aware of what you connect to them. Anything that pulls it's pin low, is not suitable for GPIO0, for example. Using GPIO2 as output activates the internal LED if set LOW.
You have to look up yourself how WeMOS maps the GPIO to D numbers. There's no logic in that.