How Many Mitel 6940 can I connect to an 8port POE switch

According to the Mitel 6940 datasheet, a standalone phone (without an extension unit connected) does require 802.3af basic PoE only (this is 15.4 W at the PSE resp, 12.95 W at the phone) what is the PoE switch in this context).

With the GS110TPv3 you referred for example, a 55 W power budget is available, you need to calculate 3x 15.4 W ... so three phones max, 46.2 W. For 4x 15.4W 61.6 W need to be calculated.

However, the phone will request more power (e.g. 802.3at PoE+ or 802.3bt PoE++) from the PoE if expansion modules are connected.

Consult the Mitel documentation FMI. Netgear has many different PoE+ and PoE++ switches available. Chime back once you have done your homework and can share exact Mitel Phones and Extender exact models, ideally with link(s) to the Mitel product(s) you intend to deploy - please keep in mind we're not Mitel (and even less Netgear, just community members).