The average 5 year old can and should be able to count as high as 30.
However not all 5 year olds will be able to count to the same number which is okay and normal and eventually the 5 year old will learn to counter higher than they currently do.
When my first son was 5 he was able to only count to as high as 20 but when he turned 6 he was able to count as high as 50.
My second son when he was 5 years old was able to count to 40.
So it can vary from child to child.
Some kids learn slower than others and some kids are faster learners.