Welcome Miguel.
Here is a way to get LOTS of Markers into a Map using Runtime Markers
Examples of simple ways to place several Markers are discussed in this forum.
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Using locations from Firebase APP INVENTOR put markers on the map from the coordinates saved in the firebase - #4 by SteveJG
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Map Tour tutorial (I think this still works) http://www.appinventor.org/bookChapters/chapter6.pdf
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MIT tutorial Map It: Displaying Locations on a Google Map
What have you tried? Are these examples useful to you?