If you are saving Tumblr URLs for later use then you are abusing Tumblr URLs. Perhaps, You haven’t read Community Guideline Point number 8 which is about Username and URL Abuse.
Tumblr Username and URL Abuse
According to Tumblr, “Tumblr’s Username and URLs are only for the use and enjoyment of Tumblr’s users. You can’t save, collect, trade or sell Tumblr username and URLs”.
You can’t register and save the URLs for later use or sell them. As per above username guideline, any username abuse can be lead your suspension or termination.
I hope you will get the whole idea why you shouldn’t save or register URLs in a large number or using automation method. If you are using them for a humble purpose then it might be OK but it depends on situation and the use of those URLs.
You can contact to Tumblr Support Team if you want to use multiple URLs on your account.