<liveAgent:clientChatQueuePosition /> in Salesforce Chat can be used to show the Queue position.

Sample Code:

<apex:page showHeader="false">
    <style>
        body { 
            overflow: hidden; 
            width: 100%; 
            height: 100%; 
            padding: 0; 
            margin: 0 
        }
        #liveAgentSaveButton, #liveAgentEndButton { 	
        	z-index: 2; 
        }
		#liveAgentClientChat.liveAgentStateWaiting #liveAgentQueuePosition { 
			display: inline; 
		}
		#liveAgentClientChat.liveAgentStateWaiting #waitingMessage { 
			display: table; 
		}
        #waitingMessage { display: none; }
        #liveAgentQueuePosition { display: none; }
        .liveAgentChatInput {
            height: 25px;
            border-width: 1px;
            border-style: solid;
            border-color: #000;
            padding: 2px 0 2px 4px;
            background: #fff;
            display: block;
            width: 99%;
        }
        .liveAgentSendButton {
            display: block;
            width: 60px;
            height: 31px;
            padding: 0 0 3px;
            position: absolute;
            top: 0;
            right: -67px;
        }
        #liveAgentChatLog {
            width: auto;
            height: auto;
            top: 0px;
            position: absolute;
            overflow-y: auto;
            left: 0;
            right: 0;
            bottom: 0;
        }
        .center {
            margin-left: auto;
            margin-right: auto;
            vertical-align: middle;
        }
    </style>
    <div style="top: 0; left: 0; right: 0; bottom: 0; position: absolute;">
        <liveAgent:clientChat>
            <liveAgent:clientChatSaveButton />
            <liveAgent:clientChatEndButton />
            <div style="top: 25px; left: 5px; right: 5px; bottom: 5px; 
                        position: absolute; z-index:0;">
                <liveAgent:clientChatAlertMessage />
                <liveAgent:clientChatStatusMessage />
				<table cellpadding="10" cellspacing="10" id="waitingMessage">
					<tr>
						<td colspan="2">
                            Please wait while you are connected to an available agent.
                        </td>
					</tr>
					<tr>
						<td>Your queue position is </td>
						<td>
							<liveAgent:clientChatQueuePosition />
							<style>
								span.liveAgentQueuePosition:empty::before { content: '0'; }
							</style>
						</td>
					</tr>  
				</table>
                <div style="top: 0; right: 0; bottom: 41px; left: 0; padding: 0; 
                            position: absolute; word-wrap: break-word; z-index: 0;">
                    <liveAgent:clientChatLog />
                </div>
                <div style="position: absolute; height: auto; right: 0; bottom: 0; 
                            left: 0; margin-right:67px;">
                    <liveagent:clientChatInput /><liveAgent:clientChatSendButton />
                </div>
            </div>
        </liveAgent:clientChat>
    </div>
</apex:page>

Chat button configuration:

Output: