Hi Colaly and welcome to the forum.
Congrats on your Monterey Jack and on moving up in size
Good question, I normally double the amount of meso starter I use just to be sure to get enough acidification and as it is cheap, if you buy a big bag from one of the Cheese Making Supply Stores
That said I use 0.25 grams of Danisco's Choozit Brand Mesophilic Starter Culture MM100 per 1 US gallon, I weigh on this scale
, it's roughly 1/8 of a teaspoon if you are in US, if metric its 0.6 ml, conversion table here