how to set seed in LTspice simulation
LT spice should be consistent from run to run. Thermal noise is not simulated in a normal transient simulation. If your seeing differences from run to run they could most likely be explained from a numerical solver standpoint and differences on rounding, which some circuits can be quite sensitive to.

Another explanation may be that the models you are using have b-sources with a .rand statement in it, but this is unlikely. 

If you are using a .rand statement, there isn't a way to set the seed (documented anywhere that I know of). If you need a .rand section to be repeated from run to run, use a PWL source, record the data as a .wav or PWL file and use that to make the runs consistent.