Currently you can set Team Names with a leading number, but the team URL cannot contain a leading number. I would suggest entering your team name as desired (eg 1testing) and the URL can be letters, eg one-testing.
Ok, the problem then is that we quickly run into the 15-bit char problem addressed by this forum post and related feature request unless we give more or less randomly generated team URLs to each channel.
A problem then is that the URL doesn’t reflect the team name in any/some way, and if we look at a scenario with 100000+ dynamically managed channels, then many probably want to be able to find a channel through specific naming, like e.g. ./PR12345-tech-discuss. It would create confusion as to whether you’d easily find a channel based on previous knowledge of URL and naming. If we start to automate more, then we would be able to determine a URL without even having to ask through some API/RESTful service for something where we may need to parse the output to be able to definitively say if this is the correct link or not.