In SharePoint 2007, is there a limit to the number of lookup-fields that you can have in a list?
No, however you might want to bear in mind the 'rule of 2000'.
Microsoft states the following:
2,000 columns per document library and 4,096 per list
But these are not hard limits they are prformace guidelines. You can see a full TechNet article on SharePoint architecture and performance limitations here