A folio number is a unique number to identify your holdings with the respective mutual fund. This unique number is like a bank account number and differs from fund house to fund house.
When you invest for the first time in a Scheme of a Fund House, a new Folio gets created and you will receive your Folio Number information when the allotment process is completed.
So let us say you invest today in "ICICI Pru MF Scheme ABC" of ICICI Pru MF, you will receive the Folio information on the next business day (once allotment i processed by AMC).
Now, further Investments in this ICICI Pru MF Scheme ABC and even other schemes, say ICICI Pru MF Scheme DEF or ICICI Pru MF Scheme XYZ can be mapped to this Folio at your option. Or you can ask for another Folio to be created.
Much like a bank account number, a folio number will reflect your holdings across different schemes of the fund house. While there is no restriction on the number of folios that you can have with a particular fund house, we recommend that you have a minimal number of folios for ease of monitoring your investments.