It depends on a number of factors: How likely is it that the city will be attacked? In the event of an attack, is there a militia? Is the standing army or a portion thereof typically stationed in the city? Is there some kind of constabulary that would do double-duty as a defense force?
Medieval military doctrine was to attack a fortified position with at least 10 times as many troops as the defender had. Conversely, if you were determined to hold a particular position, you would need to be able to garrison it with troops equal to 10% of the largest army your enemy could bring against it. I'd place that as a soft upper limit of the number of troops that should be assigned to a given city as defenders. Physical size of the city doesn't really matter, since the circumference of your walls will always be less than the circumference of an enveloping force. Thus, the defenders can typically react quickly enough to a redeployment to maintain a proper defense.
That number can be reduced by the other factors mentioned. In a city of 75,000 in which women do not fight, there are probably around 15,000 men able to bear arms who might be able to form a militia. Depending on the severity of the threat, the culture and the laws of the land, I would figure on anywhere from 10 - 20% of that number actually being in an active militia during a given conflict. So possibly 2,000 townsfolk ready to defend the walls. If you follow Ross' numbers, there is one law officer per 150 citizens, so figure around 500 for the city watch.
So then, assuming the enemy has an army similar in size to Orthigar's, and assuming they're holding about 20% back from an attack to defend their own lands, Sivan might be attacked by as many as 32,000 soldiers. We therefore want a defense force of at least 3200. We can whistle up 2500 from the populace, so we need to station about 1000 from the standing army to be certain of the city's security. Now, I believe you mentioned that there were three cities of similar size in this nation, and we want to be sure of all three of them. That leaves us with an army of 37,000. We'll need to keep some of those in reserve, about 20%, in case the enemy does something tricky. That leaves about 30,000 to use against the hypothetical besieging force. We're a little bit outnumbered, but it's our home turf, we've got the city walls as an additional weapon, and the enemy has to use some of his men to defend his supply train. Or we can go on the attack, using the reserve to pin the enemy army against our walls, giving the main army time to establish a presence in enemy territory.
Obviously, there are a lot of factors that can change things, but hopefully that gives you a decent baseline from which you can plan.