Unlimited custom fields for 4.5 came with SP 18-20. Ovsd 5.1 was frozen to SP 17 levels of functionality (or less).
Therefore the custom fields would available would be similar to that of OVSD 4.5 SP 17. I will check to see if I have the exact count of custom fields but you will not be able to create new ones once the fields provided have been used up.
They are not unlimited in SD5.1. Even worse, the number available is about 50% of the quantity we were promised. Our ability to customize the tool (at the Service Request end anyway) is completely stymied.
Don't know if HP will help you any more than they helped us. They said we shouldn't have been promised that # and that the tool was not going to be expanded. All protests about tool flexibility fell on deaf ears as they repeated their mantra: "but OVSD 5.1 is different... its not designed for that". Bla bla bla bla.
Translate that into proper english, and you get "We built a ridiculously inflexible tool, but thats ok because our client's needs are irrelevant". "Enterprise level" tool my ass. Every tool out there worth a damn doesn't consider this a problem. Service Desk Express, Heat, Remedy, even TrackIT.