I masturbate daily . Is it harmful.
Will it decrease my sperm count??

No, not harmful, but it will course other problems.

If your wife or girlfriend is trying to get pregnant, you need to stop for the week or two prior to sexual intercourse with her on her fertile days.

It can also, cause you to get too use to certain pressure and movement that does not exist in normal intercourse, which can cause performance issues

The act of daily masturbation isn't an issue. I try for that myself in addition to the rewarding sex life I have with my wife. There aren't any long term issues, with regards to sperm count or erectile issues, that you need to be concerned about. You have the consolation that you're doing this with someone you love.

As was stated above there are some other issues that you need to be concerned with:
1) Adapting and expecting a certain sensation doing sex and masturbation. IE, the death grip. Depending on how you maturbate you could condition your penis to only react to a certain sensation set. This would be an issue if your sexual partners don't measure up to that sensation. This would lead to loss of erection during sex and problems ejaculating. This can be dealt with by masturbating smartly. Make sure it isn't a lot of pressure and ensure that you're not doing any extreme sensations.
2). Sperm count can be affected. The ideal time between ejaculations for best sperm density in your ejaculate is about two days. If you're trying for a child you could lower the probability of conceiving by masturbating too often. You're not hurting your sperm production you are just using your stores as quickly as you can leaving them exhausted after each session.
3). Social stigma. Depending on what and where you masturbate it could hinder personal and close relationships. If you're happy to just wank at home, you're probably not going to look for a relationship. As well, depending on what you use to masturbate i.e. porn, you could cause strife with potential life mates because of their interpretation of what is going on.

As long as your eyes are open, you can masturbate as often as possible. Just make sure you use some lube so you don't chafe.