how old is a male dog before hes able to catch a bitch inseason ive heard its around 12 months ,and does it differ in different breeds ?

Actually he's capable from about 5 months!

will his sperm count be as high as an older dog though?

Do you mean would a few hundred thousand do as much harm as a few million? ;)

He'll be able to sire a litter, if that's what you mean.

i mean would my girl have a better chance of catching by using an older stud dog ,i have found two very good dogs one is 2 years old and 1 is 9 months old,just wondered if she had the same chance with both .

"is 9 months old"

Which one has had all the health tests carried out?

Go for the one that has passed all the breed-relevant tests for hereditary conditions. If your breed required hip-scoring (most do, except usually toy breeds and some terriers) then that can't be done before 12 months, so I certainly wouldn't use him.

Edit: You have DDB, don't you? Then certainly forget about using the puppy! What on earth is his owner doing, suggesting that it's okay to use him before his tests? Steer clear.

Even quite apart from the essential, no vital, health tests such a ship scoring that cannot be done until 12 months of age, I would never consider using such a young dog -after all it is still a pppy and has not finished growing and developing so how will you know what it will actually turn out like as an adult? It may not look anything like expected and hoped for -too large or too small for instance.

The basis for selecting a stud should never ever be based on what one would be the most successful ie in promising you a litter.

If it is a DDB then you definitely shouldn't be using the 9 month old.  As they have HD problems you should at least be ensuring that your bitch and their dogs are hip scored and have a good score.

Edited to say that of course the 9 month old is too young for hip scoring.