Yes. You can use your Subscription with as many Channels as you want - the only caveat is, the Subscription can only be tied to a single Channel at a time. So you can't have two copies of Tube Toolbox running with two different Channels using the same Subscription.
We allow you to switch your Subscription to a new Channel once every three days. For information on how to switch your subscription, please look here: How-To: Switch your Professional Subscription to a Different Channel
If you want to use more than one Channel at the same time, or not wait to change every three days, you need to purchase an additional account. If that is what you would like to do, be sure and use this link as it has a 75% discount in the activation fee - Purchase Additional Account
If you want to purchase a Lifetime Subscription for an additional account, you can use the promo code "LifetimeUpgrade" to get $20 off the additional subscriptions. If we find that you use that promo code on your first purchase, you will be banned from using Tube Toolbox.
We also have a feature called "Multi-Account Sending" which allows you to send messages and share videos from five different accounts with only one paid subscription. For more information, look here: How-To: Perform Multi-Account Sending
As far as how many copies of Tube Toolbox you can run at once on a single computer...the short answer is that Tube Toolbox was built so that running a single copy would be within YouTube's Terms Of Service. When you start running more than one copy, you are outside of their terms and on your own. We have customers who run up to ten copies at once (it really depends on the speed and RAM on your computer) but anymore more than three at once will really tie up your computer. You can find more details about running multiple instances of Tube Toolbox here: FAQ: Can I run more than 1 copy of Tube Toolbox at once?