Each custom field type has its own limits and the custom field limits are per entity. For example, In the Growth plan, you can add 150 contact fields, 150 account fields, 150 deal fields and 150 custom module fields respectively.

Custom field limit per plan

Note: 

1. If your account has 1 field each of checkbox, number, text field, and text area left unused, then the total number of formula fields you can create is 4 (even if your account has a quota of 10). Say, you have used 136 fields of type checkbox, number, text field, and text area. Although you have ten formula fields that you can add, you can only create four fields.

2. If you are creating two formula fields of return type number and text fields, then this will be deducted from your quota of number fields and text fields.

3. The character limits for the fields are as follows:
a. Text field: 256
b. Text area: 10,000
c. Number field: 11 digits before decimal point, 4 digits after
d. Default mobile number field: 30

4. You can add upto 300 options for dropdown, multi-select, radio button fields. 

Custom fields for Sales Activities

What happens to the extra fields on the downgrade?

When you downgrade your plan, so does your maximum field limit. Say, for instance, you have downgraded from Enterprise to Pro. As a Pro user, you could add up to 250 custom fields. You have added 300 fields in total. Now that you have opted for the Pro plan, your limit is 250. So, there are 50 extra fields.
" The extra fields need to be deleted"

* Please delete the extra fields before you downgrade.