How many black tea leaves are required per 125 ml of milk + 25 ml of water

That recipe actually says there are two ways you can prepare this:

There are two ways to do this: throw just about everything in a pot of water and simmer, then add the milk, tea and sweetener and brew for a few minutes; or, let the spices soak overnight in the water then simmer in the morning with your tea leaves and milk.

(emphasis mine)

Have you tried the latter? That way you still only have to heat things up once.

I'd be hesitant to boil milk with spices and then store it cold, reheating again. If you're using dairy milk, that is. If you're using anything other than non-fat (skim) milk, you'll get a "skin" on the top of the milk when you're boiling it - that's one of the classic signs of masala chai for me. But anyway, it becomes substantially less appetizing if it has time to cool into globules and is then reheated again. I'm not sure what problems, if any, you'd encounter using non-dairy milk like almond or soy.

You could dump the pre-ground spices into your brewed tea with milk and sugar, but the flavor profile will be a little different. Boiling the spices with the milk and tea gives everything time to meld nicely and provide a really smooth flavor blend. But I don't see any especially compelling reason that you couldn't do it.

I would like to point out, though, that boiling enough water for a travel mug's worth of tea doesn't take any more time to do on the stove than it does in my electric kettle. Adding milk shouldn't make too much difference; I'm willing to bet that after you get familiar with the routine, you can follow this recipe and still have it take 5-6 minutes. :)

Purists will insist that the spices must be boiled with the water, but those are the same people who would scoff at you adding cold milk to your tea in the first place. There's no reason why you can't, other than it might cool down the rest of your tea.

Hmmm..... Well, I've lived in India for 12 yrs & this is how I make 'masala chai' or 'spiced milk tea' at home & in my restaurant-

2 cups water

1 teaspoon black tea, loose leaf (I use Tokla tea from Nepal)

1 tablespoon grated fresh ginger

10 black peppercorns, coarsely ground

500 ml full fat 'whole' milk

Sugar to taste

In a stainless steel 3L pot combine tea, water, ginger & black pepper, bring to boil over high heat. Allow to boil for about 2 minutes.

Add milk to boiling tea, again allow to come to a boil. (Watch the pot closely in case the milk begins to foam, if it does foam- stir & decrease heat a little) Allow to simmer uncovered for 6 minutes.

Remove from heat, let stand for 2 minutes. Add sugar to taste if desired & stir. Use a tea strainer to strain into cups to serve. (or serve sugar on the side so people can add sugar to their own tastes)

Some notes-

I think you are trying to make 'tulsi' chai? Tulsi = Indian holy basil

If so usually the fresh tulsi leaves (around 8-10) are added to the boiling tea mixture with around 8 green cardamoms, 2 cloves, 1/4 tsp fennel seeds, and a pinch of cumin seeds as I recall. Some Indians dry roast the whole spices to mellow the flavor a bit.

Use spices like fennel, cloves, star anise, cumin sparingly as their strong flavors can take over & give the chai a 'mouthwash' taste which isn't pleasant.