Here's what I do to get the most I can from one teabag:
I use a 32oz tumbler for my morning tea; this keeps my tea hot for hours and I pour the tea into a 16oz mug for drinking, which gives me 2 mugs of tea.
I use 16oz mugs for drinking tea; 8oz equals 1 cup, so 16oz gives me twice as much.
I use a teabag 2 - 3 times in the evening with a 16oz mug.
20 bags isn't a lot, so I try to get the most I can from one bag!