MFiX can write either Binary or ASCII format data into VTP file. If you want to directly see the particle data (including number of particles), you can output particle data into ASCII format and can open with any text reader.
Try:
PRINT_DES_DATA = .T.
VTP_DIR = ‘vtp’
spx_dt(1) = 0.005
spx_dt(2) = 0.005
spx_dt(3) = 0.005
spx_dt(4) = 0.005
spx_dt(5) = 100.0
spx_dt(6) = 100.0
spx_dt(7) = 100.0
spx_dt(8) = 100.0
spx_dt(9) = 100.0
spx_dt(10) = 1.0
spx_dt(11) = 1.0