If you don't won't to see any sacking in that timber it should be designed not to sack more than a 1/300 of it's span ( in barns 1/200 or US 1/240)

Areal Momentum 2. Degree in a square timber is I=bhhh/12 (i equals d times h to the third divided by 12)

for a circle it is : i equals pi times d to the forth divided by 64

To calculate the right # depends now on the structural system the beam is in ( if the weight is being distributed evenly or concentrated in certain spots on the beam.)

For an evenly distributed weight the forces in each post A=B=force times span divided by 2

Max bending momentum M = force times span to the second divided by 8

Bening = force times span to the forth divided by 77 times elastisity# times areal momentum 2. degree

hope this helps