Q: My macbook does not detect dell wireless mouse

Without a model number, we can't rule out that the particular mouse is incompatible with OS X, especially if it is old. 

OS X pair with a lot of bluetooth devices automatically, but some need to be put in pairing mode.  You'll have to find the model of the Dell mouse and see if this is its case.

Three or four is the practical limit according to Using a Bluetooth mouse, keyboard, or trackpad with your Mac - Apple Support  Seven is the absolute maximum.  But there are other limitations.  For example, you can only connect devices of the same class one at a time.  Mouse falls into this limitation as does Bluetooth speakers.  One at a time for these items.