If your breed society uses normal ID tags, then the next consecutive numbers will be issued to you automatically when your tags are printed. There must be a central database to be used by all tag producers, so no matter which company you use you will still get your consecutive numbers allocated.
Some breed societies issue breed-specific tags, which are numbered according to when the lambs were registered, so again you have no input to the numbers you receive (with this type of tag you still have to put in a CPH tag).
If your breed society requires additional information to go on the tag then ask for that with your main order. There will be some discussion as to how many pedigree tags and how many routine tags you want, but phone the tag company to discuss it. I think most tag companies are geared up for this sort of problem, so just keeping the company you have been with is fine.
This is my understanding of the situation.