Can I hold more than one ISA?

Yes.  Providing you are eligible you can invest in each type of ISA.  As ISAs do not close when the tax year finishes, you can also hold multiple ISA accounts.   

However, you can only put money into one of each kind in each tax year.  So if you have put money in one type, you cannot open a second of the same type.   For example, if you have put funds into one IFISA in the current tax year, you cannot put funds into another IFISA unless you transfer the account.   

There is an exception to this for Flexible ISAs.  See the question 'What is a Flexible ISA?'.