You will keep the number of X variable observations the same in regression. Since increasing the number of X variables likely increases SSR and R² why not include every variable you can obtain in the regression equation?

Yes, you can include as many significant variables as you wish since computation power is the only limitation on regression modeling.

No, since the F statistic and Adjusted R² value degrees of freedom increase and they decreases as a result.

No, since coefficient t-values will decrease as more variables are added.

No, since the accuracy of the additional data decreases as more variables are added