Values of A,K,N,a in Cobb-Douglas function expressing GDP
K is just a stock of capital. This Fed data show that the stock of US capital is almost 70m at constant prices for the most recent year (2019). But there are other ways of measuring it. You can google papers that estimate multifactor productivity to see what measures for K and N people are using (not everyone uses labor hours too).
A is multifactor productivity. This OECD data show that using 2015 as index year the multifactor productivity in the USA in 2019 was 102. A cannot be measured directly. People estimate it using K and L and using time series/panel regressions. That is why this will be in datasets expressed just as a growth rate or index, we can't see what A is only how it changes.