When I'm trying to signing in my iCloud on my iPhone it is showing maximum accounts have been activated on this device. What should I do?

Sign into iCloud using that Apple ID on another device/computer. You should then be able to sign in.

So, it doesn't sound like you are trying to sign in. Are you going to the iCloud.com site? If so, then unless you request the desktop site, you have no way to sign in, and the option you are selecting is indicating to the system that you are trying to activate a new iCloud account.

Cheers,

GB