(float, 1 integear + 5 decimal digits)
That is not the definition of a float as a string. There could be one character before the decimal point, and 5 characters after the decimal point.
Reading 8 characters will be very fast. Receiving 8 characters will take time. How much depends on the baud rate. Receiving, reading, and saving 3200 characters to the SD card every 10 seconds might be possible, with a high enough baud rate. You will need to try it.
You don't NEED to remember more than 1 character at a time. The HardwareSerial and SD classes will take care of remembering the rest.
Reading 3200 characters, converting them to floats, and writing them to the SD card, every 10 seconds, isn't going to happen.
What is the max number of variables.
Divide the number of unused bytes of memory by the size of one variable to get the number of variables you can have. We don't know what your sketch is doing, so we can't tell how much free memory you have. We don't know what type variable you want to create, so we can't do the math for you.