For that many large lemons, make a liter of simple syrup. You will have a very strong syrup that can be diluted with water.
Taste it after diluting. The zest should sink to the bottom of the jar and try to leave it undisturbed, otherwise you will need to filter it if you are going to use it in drinks or ??. It doesn't matter if it is going into a marinade or other mixture that includes herbs and spices.