A simple for loop iterating by one can be done by using the range
for (i in 0:10)
print(i)
To increment by two you can use the seq function
for (i in seq(0, 10, 2))
print(i)
This will print out every second number, since it is iterating by two here.
Source: https://www.rdocumentation.org/packages/base/versions/3.6.2/topics/seq