How should I use intumescents when passing pex through fire wall?
First, this is a fire separation, not a true firewall
California Residential Code R302.6 (and the IRC it is based on) is clear on this point (italics in original):

R302.6 Dwelling/garage and/or carport fire separation The garage and/or carport shall be separated as required by Table R302.6.  Openings in garage walls shall comply with Section R302.5.  Attachment of gypsum board shall comply with Table R702.3.5.  The wall separation provisions of Table R302.6 shall not apply to garage walls that are perpendicular to the adjacent dwelling unit wall.  A separation is not required between the dwelling unit and a carport, provided that the carport is entirely open on two or more sides and there are not enclosed areas above.

So, you don't need to go crazy on this
From there, we chase down the reference to R302.5.3 on &quot;other penetrations&quot;:

R302.5.3 Other Penetrations  Penetrations through the separation required in Section R302.6 shall be protected as required by Section R302.11, Item 4.

Item 4 of R302.11, in turn, states:

Fireblocking shall be provided in wood-framed construction in the following locations:
...

At openings around vents, pipes, ducts, cables, and wires at ceiling and floor level, with an approved material to resist the free passage of flame and products of combustion.  The material filling this annular space shall not be required to meet the ASTM E136 requirements.


As a result of this, you don't need intumescent firestopping collars around these pipes at all, just something there (usually rockwool and a fire-resistive caulking) that will keep smoke and flames from slipping by around the pipe openings.
Since you're using intumescent collars, though, I'd use one collar/penetration per pipe -- since they work by expanding and pinching off the pipe, one-per-pipe is the only way to install them so that they'll work properly.