It's not illegal to buy as many as you can get. However, you have to declare them to the other life insurance companies on the application, and, each usually has a "maximum total of policies".
Also, the "short period" your father has to live through in order to collect, is either two or three years.
So, if he lives for at least 24 months after buying the policies, they should pay out a full death benefit, subject to any exclusions. in the policies.
Here's how you can learn more about guaranteed acceptance life insurance and apply for a policy online.