Multiple validation problem

I'm working a sheet which we use at work (a restaurant) to record our reservation.

I have provided an example sheet to explain what I'm tyring to do.

If a guest wanted to book a table at 5.30pm for 2 people, I would enter a '2' into B4 (or whaterever time they wanted/free table). I have then set up a conditional format to shade a pre-defined number of boxes to signify how long that particular table will be booked for. (as you can see from my example text in L11 (shading done manually however as I haven't finished the conditional formatting for the entire sheet)

The problem I have is that I have set validation rules for the tables so that they cannot be booked over and above a set number of cover. The problem I'm encountering is that in the next cell along from the inserted number I would write the customer details (again see M11) but because this text is not defined in the validation rules it through back an error.

Is there a way to set two type of validation rules for one cell?
What I would need is to validate the numerical data inserted, but also allow for any text to be typed without the eroor showing.