The included ContourXD background mapping has 3,000 bodies of water nationwide with contours. That is tens of thousands not included.
Humminbirds read Navionics and Lakemaster map products. The best one for you is the one that has the data you want, where you want it, how you want it.
You can download the Lakemaster list from the Humminbird Lakemaster website. The ones listed as Lakemaster High Definition Survey are very good, the other two types, not so much.
You can view the current Navionics maps on their website's WEB APP.
When you view that, there is a circle at the lower left of the panel. Click on it to change the color after you have zoomed some. When the circle is grey, that is Navionics data. When the circle is blue, that is Sonar Charts which is HD mapping created from uploaded sonar logs by unit users. You cannot purchase Sonar Charts, that is FREE to download to a Navionics+ card or Hotmaps Platinum card.
Humminbird Helix 12 SI, 1198c SI, 1198c SI, ONIX 10 SI NT, 360, RC-1