Phone SIP accounts vs number of SIP trunks

[bell.b@harrisburgacademy.org]

Good morning.
What is the relationship between the number of SIP trunk lines coming in to a PBX and the number of SIP accounts a phone has? In my business, we are looking into replacing our old digital PBX / eight analog POTS lines with a VOIP PBX (like Asterisk or 3CX) and SIP trunks. I have already made the assumption that we will have eight SIP trunks to replace the 8 POTS lines to maintain the same number of simultaneous incoming/outgoing calls. However, in looking at various IP phones to use with the PBX, they have varying specifications for number of SIP accounts. Some are as low as one, and other (more expensive) phones have upwards of 15 or 16. Do I need the more expensive phones to handle all eight SIP trunks, or will a three SIP account phone work and not limit it to accessing only three of the eight lines? I suspect that it will, since some companies could have hundreds of incoming SIP trunks, yet I have never seen a phone with that many SIP accounts.

Thanks for all your help.

Bob