Stargazer is really nice package however sometimes you have to do some tweaks "by-hand". If you want to have number of observation in you output this is how to go about it:
stargazer(mod1,
type="text",
add.lines = list(c("n", nrow(df), nrow(df))))
if you want to create a table in latex you can use:
stargazer(mod1,
type="latex",
add.lines = list(c("\\textit{$n$}", nrow(df), nrow(df))))
This approach is so that once you render it in latex "n" will be in mathematical font.
==============================================
Dependent variable:
----------------------------
B C
(1) (2)
----------------------------------------------
var2 0.0002 -0.055
(0.080) (0.079)
var3 -0.088 0.012
(0.078) (0.077)
Constant -0.029 0.030
(0.078) (0.077)
----------------------------------------------
n 1000 1000
Akaike Inf. Crit. 2,206.078 2,206.078
==============================================
Note: *p<0.1; **p<0.05; ***p<0.01
Hopefully, this is what you have been asking for.