Hi Harold,
Transferring routes is not one of Mobile PC's strongest features at the moment.
As you have discovered, the current version of Mobile PC will only accept one route at a time. It has been reported to Garmin and accepted as a defect.
You can actually transfer multiple routes and if they have unique names you will see all the different names in the "Manage My Data > Routes" list. However, they will not be all of the different routes. They will actually be the detailed routing for the first name in the list for all of the routes in that transfer. That is, Mobile PC picks up the individual names okay but it attaches just the first route details to all of them and ignores the rest.
There is a limit to the length of the route name Mobile PC will accept. According to Garmin Product Support it is 35 characters. Your guess is as good as mine what might happen if your route name in Mapsource is 36 characters or longer when you try to transfer it to Mobile PC. ... Especially if the first 35 characters are not unique.
Check in Mapsource and see if the route names are 35 characters or less and that they are unique. That may not be the problem but it's a place to start.
If that doesn't fix the problem, would you mind posting some of the problem routes here so we can take a run at them? (Just attach them to a message with a quick description in the message of what happens when each one is transferred to Mobile PC.) The original GDB file would be best (strip out anything you don't want to share but make sure the sample route(s) still break when transferring to Mobile PC before attaching the file).
You can easily attach files by clicking on the "Manage Attachments" button down below the text editing box in the section titles "Additional Options". Ooops, GDB is not listed in the permitted file types, so you have a couple of options. Either just zip the GDB file (prefered) or save it as a GPX file. Either .ZIP or .GPX will upload just fine.
...ken...