If problems are many, how many devtas can be activated at one time?
In a vastu pooja all 45 devtas are activated together in one sitting. A single devta can also be activated on its own with its samagri at its own time.
Related questions
Are there 45 vastu devtas or 49?
There are 45 devtas, but 4 more devtas stand on all four sides to protect them, so basically there are 49. Bhog samagri is offered to these 4 guardians as well, after the 45.
Do the Vastu Devtas reside in the sky or on the ground?
On the ground. That is why they should be established at the shilanyas itself, and their bhog samagri is offered on the ground at each devta's own place.
Does the relationship with the Vastu Devtas end once the vastu pooja is done at the start of construction?
No. A farmer who sows a seed does not end his bond with it there: he protects, waters and nurtures it until it flowers, fruits and grows into a shady tree, even a kalpavriksha (wish-fulfilling tree). In the same way, Indian architecture holds that a structure of stone and lime is not lifeless; prana settles into the building and its soil and controls the feelings of the people who live there, so the devtas established in the land must keep being nurtured.
Does the time of offering matter when worshipping the devtas?
In a full vastu poojan all 45 devtas are worshipped and offered together. When one particular devta has to be activated, offer its bhog samagri or do its jaap in its own time slot for the maximum result. An offering at another time is not useless, but just as food eaten at the fixed time of hunger satisfies more, the result differs.
If many things are already kept at a devta's place, does that count as an offering?
No, goods lying there are not arpan or samarpan. A guest never picks sweets off the dining table himself; they are served on his plate and he is invited to start. In the same way, serve the devta its bhog in a leaf dona or on a paan leaf, which the devtas love, and request it to accept the offering and increase the energy of the place. If the pooja was not done during construction, offer it at griha pravesh: any dish made in desi cow ghee, such as gujiya.
When do the Vastu Devtas have the greatest effect, and when should they be established?
Students answered griha pravesh and bhoomi poojan, which is only partly right: the greatest effect comes at the garbh vinyas or shilanyas, the day the land is taken over and the foundation is laid. Then the pishach, manushya, dev and brahma bhoomi are marked, the vastu pooja is done and every devta, 32 in the pishach bhoomi and the rest inside, is worshipped at its own place with its samagri, following the vidhi. A child cannot be born without a seed in the womb, and food works only through the mouth, not the ear or nose, so the biggest pooja, havan and sthapana belong to that day; building over the devtas afterwards is like tethering a cow first and then building its shed. Done later, remedies bring back only about 35% of that role, though even 5% is a lot.