Samaritans number at most, 1,000-2,000 people.
The biggest difference between Samaritans and Israelite belief is that Samaritans believe that the “place that God will choose” mentioned in Deuteronomy is Mount Gizrim in modern day Palestine/West Bank, while Jews believe it is Jerusalem. Because the “place that God will choose” is the only place Israelites can offer sacrifices but the Jewish “place of God will choose”–i.e the temple mount in Jerusalem–is inaccessible only Samaritans still offer sacrifices. That is the biggest “theological” difference–where is the “place that God will choose.”

The Jewish and Samaritan Torahs are almost identical. Samaritans consider the Torah/Pentateuch (their version that is) holy. They also have post Torah writings, including a book of Joshua and other books explaining post Israelite conquest history. The Samaritan Torah is very similiar to the Jewish Torah, here are some notable differences:
The blessing of Judah at the end of Genesis adds, after noting that the sceptre will never depart Judah, that it in fact, will depart Judah: “until Shiloh come; and unto him shall the gathering of the people be.”

The “bridegroom of blood” from Exodus chapter 4 story is more metaphorical (Tziporah has to ‘circumcise her heart’)

The Samaritan ten commandments (in Exodus at least) have a tenth commandment of setting up an altar on Mount Gizrim.
Two significant variants are Exodus 12:40 and Genesis 2:2. The MT reading of Exodus 12:40, confirmed by the DSS at 4QExod(c), says that the Israelites spent 430 years in Egypt alone, while the SP (and the LXX) read that 430 years includes time in Canaan (cf. Galatians 3:17, Ant. 2.15.2). The significance is discussed in this post. Genesis 2:2 in the MT says that God ended his work on the seventh day, while the SP–and once again the LXX too–says that God finished working on the sixth day, to avoid the appearance that God did any work on the Sabbath. Rabbi Dr. Zev Farber gives a nice summary about the SP:
The Samaritan Pentateuch is often referred to as a full or expansive version. The reason for this is that one of the key differences between the SP on one hand and the Masoretic Text (MT) or Septuagint (LXX) on the other is that various editors responsible for producing the SP smoothed out many of the rough edges in their Torah by adding or revising text.
In his article, Rabbi Farber gives examples of the SP’s revising the plague narratives: https://thetorah.com/the-missing-speeches-in-the-plague-narrative-and-the-samaritan-pentateuch/