How many is/are incorrect?
(1) Isoelectric point is the pH at which an amino acid exists primarily in its  neutral form.
(2) Isoelectric point is the average of $p{K}_a$, values of  $\alpha$ -COOH and amino  $\alpha$ -  ${\mathrm{NH}}_3^{+}$groups [valid only for neutral amino acid]
(3) Glycine is characterized by two $p{K}_a$ values.
(4) For neutral amino acid, the concentration of zwitter ion is maximum at its  isoelectric point.
5) High density polythene is a linear polymer
6)  Low density polythene is a branched chain polymer
7) Chain growth polymers are also known as addition polymers.
8) Step growth polymer is also known as condensation polymer.