Transmissions need the proper amount of fluid to function correctly. Overfilling them can create stress in seals, and not having enough fluid can create a situation in which the gears are not being lubed enough. Every vehicle has a transmission fluid specification for the proper amount--even standard transmissions. Checking the fluid level regularly is a great preventive maintenance procedure to ensure your transmission is operating properly and has the correct amount of fluid to do so.
How Much Fluid
To ask a question to a question, the proper amount of fluid to add to the transmission depends on why you're adding it. Most passenger vehicles take from 12 to 16 quarts of transmission fluid, but the model of the vehicle dictates the type and how much. While many vehicles come with a dipstick to check the transmission fluid, there are other models that do not. These cars require lifting levelly and checking the fluid from underneath the vehicle. While performing a transmission service, where the pan is dropped and the filter replaced, 4 to 5 quarts of transmission fluid should be adequate to replace what was lost from the pan. If you continuously have to keep adding transmission fluid due to a leaking seal, the vehicle should be inspected by a qualified transmission technician.
How and When to Add Transmission Fluid
Vehicles with dipsticks can easily be filled through the dipstick by using a funnel and pouring the fluid in slowly. The vehicle should be warmed up to operating temperature and parked levelly while in park or neutral to properly check the level. The reason to warm the car up first is to allow the petroleum-based fluid to expand to its maximum level. As mentioned, vehicles without dipsticks will require lifting the vehicle so it is level. Finding the fill plug will also be required, and much like a differential plug, you would add enough fluid until it seeps lightly from the fill plug port hole. Sealed transmissions, without a dipstick or fill plug, should only be serviced by qualified transmission technicians.
Flush vs. Pan Drop
You've dropped a pan, emptied out the old fluid in the pan and then replaced the filter. Great. Understand that when you put the pan back up and then replace the fluid lost from the pan, you're going to be pumping the brand new 4 to 5 quarts of transmission fluid through 8 to 11 quarts of old, dirty transmission fluid located in the torque converter--unless you're lucky enough to have a drain plug located in the torque converter. Pan drops may be old school and a common practice in the auto maintenance department, but since the introduction of transmission flush machines, this procedure is worth its weight in gold. Far superior to pan drops and filter replacements, many modern transmission flush machines purge all of the old transmission fluid out using only the pressure of the vehicle's transmission. Many claim to even clean the filters internally. Although much more expensive than pan drops and filter changes, you get what you pay for.