Yes, every 10 to 14 days with the 1 mm size.
Dermaneedling works without any creams but the creams help establishing good conditions in the skin for collagen production.
The skin on the knees is very thick and a 1.5 mm size is appropriate. Also, you should not use the same dermastamp on your face and on your knee because the tough scar tissue on tough knee skin will make the dermastamp blunt sooner and you need a sharp dermastmp for your facial scars.
Concerning the hyperpigmentation and Ambi cream, it depends what is the cause of the pigmentation.
Sometimes, it is residual inflammation. Try this: buy a non steroidal anti-inflammatory such as Nurofen rapid capsules (it contains ibuprofen). For sale in pharmacies (OTC). Prick the capsule and apply some of it at least twice a day on the pigmentation. It may sting.
And sometimes it is overproduced melanin pigment. Melanin is skin pigment that is normally present in the skin but unfortunately the skin often reacts to skin injuries by overproducing the pigment.
Even very small skin disturbances, like a bug bite, can in some individuals result in the overproduction of melanin in that spot. What helps then is Tyrosinase inhibitors such as hydroquinone. They inhibit the enzyme that converts tyrosine to melanin. Ambi fade cream contains 2% concentration of hydroquinone. Use your dermastamp to enhance the penetration of hydroquinone or the anti-inflammatory but do not insert the full length of the needles for that. Just the tips so that you prick the outermost layer of the skin. Once in three weeks, stamp the scar with full length of the needles to crush the scar tissue.
It can also be a combination of residual inflammation and melanin overproduction.
What often helps (in both cases) are products that speed up the turnover of the skin - mcironeedling, acid peels, Tretinoin cream etc.