how many routers can i set with one hostname? If yes, will they conflict?

You can, but the result will be that when either router has a changed WAN IP it will update the hostname with it's IP address. This means that the hostname will resolve to one or the other router (apparently) randomly.

What you should do is have one hostname for each separate Internet connection.

Another thing is if I have a dyndns account and I have two hostnames, can I set them both to two seperate internet connection?

You can set each to a separate connection, but you can't use both on both connections.

Can i set one hostname to two different routers with different ISP?

If this is for a Dynamic DNS hostname, then not at the same time, but only to one router at a time. So no.

If this is for an own Dynamic DNS domain name, then yes, you can configure more than one A record for it, each pointing to a different IP address. Not sure if this helps with what you want to achieve however.

Thank guys, thank you so much.

Another thing is if I have a dyndns account and I have two hostnames, can I set them both to two seperate internet connection?

Thank you

Calvert