Is there any limit of maximum number of custom fields for any custom post type?

I found 2 weird thing when I build custom fields for a custom post type.

1. I created a new "Custom fields" which location is set to a newly created "Post Type" which is named as "PlasticSurgery". Then, I was building fields for the "Custom Field". To some certain number of fields, everthing worked fine. However, when I try to add a new field, the location setting of the "Custom Field" is automatically set to the wordpress standard post type. Even I tried to set its location back to the "PlasticSurgery", it is impossible.

2. I ignored this weird behavior and continued to add more fields, but I found I can not add more field.

Would you let me know if there is any limit for the number of fields for a single custom field?
I am sorry but I am not a developer and I'm afraid that metabox is too difficult for a non-techy like me.