As Goose52 stated, you don't need to be concerned with using up all the memory capacity.
No, the card will not overwrite after it is full, it is a map card and can be full eventually. IF you ever fill it up, that's it. Another card is in your distant future.
It has about 14 GB of water body shorelines (Zero Lines) and the rest of the 32 GB capacity is for the contour map creation inside the Zero Lines. If you got the right card, it states "North America", there is a body of water or three in that area of the world. Promotional data states "millions" of zero lines.
The Zero Line card I have is not a SD card. It is a microSD inside a SD adapter. It has that size card for use in the Helix 5 and Helix 7 models with microSD readers. The data has to be processed by either the Auto Chart or Auto Chart Pro computer software to use in those units. The Zero Line card is not exclusive to Auto Chart Live. When you purchase the computer software, a Zero Line card comes with them.
Maps created by Auto Chart Live and Auto Chart computer software function like Lakemaster mapping in the units.