Where do you fill the transmission fluid on a 98 Nissan Altima?

Fill it in with thin funnel in the same place as the dip stick.

how many quarts of tranny fluid does the 98 altima take?

I believe 4 quarts. When in doubt, put two quarts in, and check the fluid level and add until full. If you don't know how to check transmission fluid, park the vehicle on a level surface, start engine and move transmission through all gears (Park, reverse, drive, etc.)

Put in park and check fluid level with vehicle running.