how many monitors the new imac 21.5 drives

I gave you no link to an advertisement, but rather to Apple's technical specifications page for the 21.5 inch iMac. That details the types and resolution support for external display support. That iMac model has two TB3/USB-C ports on it and does not restrict you from using both within those external display guidelines.

See the video support and camera section for the 21.5 iMac here.

Depending on the monitors you choose and their resolition, yes it can support 2 extra monitors. Here are the specs from the link above,

Simultaneously supports full native resolution on the built-in display at millions of colors (21.5-inch) or 1 billion colors (21.5-inch 4K) and:

One 5120‑by‑2880 (5K) external display at 60Hz with support for 1 billion colors, or

Two 3840-by-2160 (4K UHD) external displays at 60Hz with support for 1 billion colors, or

Two 4096‑by‑2304 (4K) external displays at 60Hz with support for millions of colors

Thunderbolt 3 digital video output

Native DisplayPort output over USB‑C

Thunderbolt 2, HDMI, DVI, and VGA output supported using adapters (sold separately)