I am resuming my piano in a couple of weeks. I have bought a 61 key weighted key keyboard
How many grades will I be able to do before I have to up it to a 76 or an 88 key?

Very soon, don't even waste time with a 76.

If you are still doing twinkle twinkle little star, depending on ability, a 61 key may last a year, some students will outgrow it in 6 months, others may take 2 years.

If you had not already bought, it, professional opinion, waste of money.

If that is all you can afford, it is better than none.