Multiple resistances only count once (by multiplying effective hit points)
If a monster has resistance or immunity to several damage types-
especially bludgeoning, piercing, and slashing damage from nonmagical
weapons-and not all the characters in the party possess the means to
counteract that resistance or immunity, you need to take these
defenses into account when comparing your monster's hit points to its
expected challenge rating. Using the Effective Hit Points Based on
Resistances and Immunities table, apply the appropriate multiplier to
the monster's hit points to determine its effective hit points for the
purpose of gauging its final challenge rating. (DMG p. 277)
From the text above, giving the monster multiple resistances means they recommend that you multiply the effective HP by an amount determined by the expected CR of your monster.
So in other words, if you give your monster multiple resistances, you will multiply the effective HP of that monster based on the number in the table on p. 277 which requires you to estimate the CR of the monster you are creating. It doesn't seem to matter how many resistances you give it (as long as that number is greater than 1), the multiplier doesn't change for a given range of CRs for your monster.
This may seem a bit counter-intuitive given that you are trying to find the CR for your monster, but generally you should at least know the range you want the monster to be when you are creating it based upon the party level and composition etc. This result will then factor into the defensive and offensive challenge ratings for your monster which will determine its final CR as outlined in the rest of the "Creating Monsters" section of the DMG.
It is also worth noting that you need to take into consideration the party's ability to bypass these resistances when calculating the CR as mentioned also in the text excerpt above. For example, if your party has magic weapons, nonmagical BPS resistance means nothing.