I've sampled at 8 times on one job and it was OK so there is no rule dictating that 16 is the magic number so sampling higher than this is not a problem in anthing other than handling bigger numbers in hardware. Here's a the 16x counter idea: -
From the falling edge of the start bit you "find" the start-bit's "middle" by counting to 8 then, each count of 16 thereafter you "sample" the UART received data to recreate the byte (or bytes) transmitted. Clearly if you had a 32x counter you'd get a tad more accuracy in determining the centre-point of the bit and counting at a higher rate is going to work but the numbers get bigger and the power consumption rise.