keep certain number of backups with windows server backup

I'm trying Windows Server Backup, and it's doing incremental backup in order to save disk space.

it's working greate, but now I have 36 copies (1 is full backup, others are incremental changes since the full). is there any way to keep only last 30 days backup? or it's impossible for this kind of backup

Not possible with the way you're doing it. You need all the incremental backups since your last full if you want to be able to restore.

Incremental backup. An incremental backup backs up only those files created or changed since the last normal or incremental backup. It marks files as having been backed up (in other words, the archive attribute is cleared). If you use a combination of normal and incremental backups, you will need to have the last normal backup set as well as all incremental backup sets in order to restore your data.

If you want to purge your incrementals more frequently, you need to do a full backup, then purge the older backups.

And, incidentally, it doesn't sound like you have a good backup strategy. Backups are really only useful if you have some level of retention on them. What happens if/when you find out that someone deleted an important file/your server got hacked/your filesystem got corrupted 3 months ago? If you're only keeping backups from the last month, you're screwed, is what.

Also, I sincerely hope you're not keeping these backups on the same disk(s) as are running the server, because if you are, they're not really backups.