I am thinking to transfer all my domains to one registerer so that the control will be easy.
But I am a little bit worried about the following question: what would happen to my domains if my domain provider company bankrupted?
Simply management for the domain would revert back to the central registrar such as ICANN or Nominet. You would then be able to transfer your domain to another registrar. Your ownership and use of the domain would be unaffected.
Prior to that final end, your registration contract would probably be considered an asset and sold to another registrar. An example case would be when RegisterFly (Wikipedia article) went under, many of their domain registrations were sold on to
I think you might experience a big downtime to all your domains/sites because domain provider usually do not only register domains for you, but they also provide Name server/DNS that customer (like me) usually use through a control panel to change DNS records. Even if the domains would be transferred to ICANN and you definitely won't lose the property on them, the would need new Nameservers/DNS and I don't think it's something that happens immediately, there must be some other domains provider that buy/take them.
But thinking at it from a business perspective,I would say that it's quite difficult for a domain provider to go bankrupted. Even if business go bad for him, he would probably just sell all its domains to some bigger provider and after that close the company. In this case you would probably not experience any downtime (maybe a very small one) because domains would be all transferred to the new domain provider Nameserver DNS before the old company closes (I'm not a business expert, but selling domains usually do not require big investments/debts/loans).