We all are pleasure seeking entities. We want happiness. Without following rules(Niyama) and Regulations(Nitya Karma) there cannot be any real happiness in life. This is confirmed in Bhagvad-Geeta(3.10).
In the beginning of creation, the Lord of all creatures sent forth
generations of men and demigods, along with sacriﬁces for Viṣṇu, and
blessed them by saying, “Be thou happy by this yajña [sacriﬁce]
because its performance will bestow upon you everything desirable for
living happily and achieving liberation.”
These rules and regulations are already described in different sections of Vedas. The importance of following dharma is stressed again in following verses:
All living bodies subsist on food grains, which are produced from
rains. Rains are produced by performance of yajña [sacriﬁce], and
yajña is born of prescribed duties.
My dear Arjuna, one who does not follow in human life the cycle of
sacriﬁce thus established by the Vedas certainly leads a life full of
sin. Living only for the satisfaction of the senses, such a person
lives in vain.
This second verse quoted above mentions the fate of those who don't follow these rules and regulations.
In general people are reluctant to follow these rules and regulations simply because they find them to be hindrance to their freedom. But Acharyas such as Srila Prabhupada calls these rules as "rules and regulations of freedom".
To experience the higher degree of freedom we have to voluntarily give up some freedom; i.e. follow these codes of conduct mentioned in scriptures even though we might have aversions for them.
An Example may be taken of driving on road. Eventhough we are free to drive a car on road;we have to follow the rules of traffic; otherwise the freedom to drive(in case of accident or some legal action) will be totally taken away from a person.
So those people who do not follow the cycle of sacrifice is put into helish condition of life; such as life of an animal,insect,tree etc. wherein the freedom is totally taken away and the living entity becomes a puppet in the hands of material nature.
hence in 2nd chapter of Geeta(2.40) while speaking about working without fruitive desire Krishna Says:
In this endeavor there is no loss or diminution, and a little
advancement on this path can protect one from the most dangerous type
Here "most dangerous type of fear" refers to a fear of being cast into hellish form of existence as I've mentined above.
regarding your 2nd Question:
Also practically how many of them can be followed individually for a samsari?
Vedas in them self are huge and describe number of rules and regulations to be followed by person desiring to achieve a particular goal(including Liberation from material world). In addition there are books such as Manu-smriti which outline the codes of conduct for human society.
It will totally vary from person to person as to how many of these rules he can follow.