It worked, but after I made it as store=True the count of all sale order was reset to 0. Why is this problem?

Hi Moe,
Thanks for giving vote for the answer.
Yes, Now your function will call once you change your function dependent fields and calculate the result . Integer field type has the default value 0.

Can you give me a glimpse of the code please, I didn't get it so much. You are welcome Malay

Yes sure Moe,
Please try with this

sale_order_count = fields.Integer(compute="_get_sale_order_count", string="Sale Order Count(s)", store=True)

def _get_sale_order_count(self):
for rec in self:
rec.sale_order_count = len(rec.sale_order_ids.ids)

where sale_order_ids is the field inside your model, or search all the sale order ids and add it instead of the len(rec.sale_order_ids.ids)

Please vote up the answer if it helps you.
Thanks