Do we know that exact number of the armies at the Trident? I seen to recall Robert having 35k and rhaegar having 40k. If this is true doesn’t it seem a bit low for the combined strength of the Starks, arryns, tullys, and baratheons?

It’s not their full strength – Eddard would have had to rush south like Robb, the Vale was badly divided and would have needed occupying forces, ditto the Riverlands, the Stormlanders were divided and took significant casualties – but you’re right that the numbers are low. 

Between the four houses, you’re looking at a full military strength of 110,000 (35k for the North, 30k for the Vale, 20k for the Riverlands, and 25k for the Stormlands). Even accounting for division and casualties, 55k is more reasonable.