I've had good luck with touringplans (which rates crowd levels by attraction wait time, not strictly by numbers of guests, which I prefer since wait times are what I really want to know), but crowd level forecasts can change right up until the last minute due to park hours changes, shifts in entertainment schedules, and even weather. Use them only as a rough guide. A good touring plan can largely overcome those little crowd fluctuations anyway, so unless the crowd estimate difference is huge, choose dates and parks in the order that works best for you, and don't sweat the rest too much!
[crowd projections and touringplans subscription recommendation omitted to appease member below]