AFAIK there is no hard limit. However, you should avoid having too much data (many services created by one template with many fields). If you reach a point where you have to ask how far you can go with input fields you probably try to build very complex templates or services that are very large. Might be worth considering multiple service templates to reduce the number of input fields/complexity or you can look into scripting the CRUD actions of services through automation scripts that immediately build a service instead of going through a service template.