Data plans are offered in 300 KB, 1 MB, 10 MB, 100 MB, 500 MB, 1GB and 5 GB.
Each SIM card requires it’s own line, but multiple lines may be under one plan as long as they are of the same size. Allowing multiple lines under one plan assists in avoiding overages, as all lines on the plan jointly share the data.
Example: I have 5 SIM cards. I purchase a 1 MB line for each SIM card. Total this plan is allotted 5 MB of data. This allows me to have card 1 use 2 MB, card 2 use 1 MB, and card 3-5 use 500 KB each. The total used is 4.5 MB, and card one doesn’t get overage charges, because it pulls the extra data from cards 3-5.