Collection numbers are automatically assigned. However it would be desirable if it was possible to:
First sort by type (DVD, BLURAY etc (feature already exists).
Then select the titles of the collection that I want to renumber, enter a "Beginning Collection number" on the first selected title and let the system renumber the selected titles.

Example: I have Blurays and DVDs. First I sort them by type. Now all the DVDs are together in one block and the BRs are together in another block. Their numbers however go all over the place.
Next, I select the DVDs, assign a starting number (example 001) and let the System renumber the DVDs. Let's say the last DVD number is 246.
Now I select the Blurays, assign a starting number again (example 300) and let the System renumber again.

randronoth

Not really solving your problem, but maybe a completely different approach that would yield a equivalent result:
Have a look at Collection Categories. By putting DVDs and BRs into different Collection and give them each their own Collection Number.