Degrees of freedom F test
Find the degrees of freedom for a particular F test below:
Test  Degrees of freedom numerator  Degrees of freedom denominator 
One way ANOVA $F$ test  $I  1$. Here $I$ is the number of groups.  $N  I$. Here $N$ is the total sample size and $I$ is the number of groups. 
Two way ANOVA $F$ test 
$F$ test for main and interaction effects together (model):

$N  (I \times J)$. Here $N$ is the total sample size, $I$ is the number of groups for independent variable A, and $J$ is the number of groups for independent variable B. 
$F$ test for complete regression model (OLS)  $K$. Here $K$ is the number of independent variables in the model.  $N  K  1$. Here $N$ is the sample size and $K$ is the number of independent variables in the model. 