Google Maps' default behaviour is to place a new pin every time you do a search. However, in doing so, it erases your previous search pin. To place several pins in Google Maps, you have to create a custom map. From there, you can keep adding new pins without the old ones vanishing.
Open your Internet browser and go to Google Maps.
Click "My Maps" on the left.
Click "Create new map."
Give your map a title and description, then click "Done."
Search for the first place you wish to pin on your map in the search box at the top of the page.
Click "Save to..."
Select your map name from the drop-down box and click "Save."
Perform additional searches for other pins you want to place on your map. Choose "Save to," select your map and click "Save" for each pin you want to add.