JSFiddle - React, Tailwind, and code Playground

HTML

<table id="mytable">
<tr>
                <td>1  </td>
                <td>1</td>
        <td>ЯСИНСКАЯ-3 И. В</td>
                <td>СТА6220253647</td>
</tr>
<tr>
                <td>2  </td>
                <td>2</td>
        <td>ГОРБАЧЕВ Н. Н       </td>
                <td>РОС2030253182</td>
</tr>
<tr>
                <td>3  </td>
                <td>3</td>
        <td>ХОЛОДНЯКОВА С. С</td>
                <td>РОС2030253183</td>
</tr>
<tr class='hideit'>
                <td>4  </td>
                <td>4</td>
        <td>nbsp;ГОРБАЧЕВА В. Г</td>
                <td>РОС2030253184</td>
</tr>
<tr class='hideit'>
                <td>5  </td>
                <td>5</td>
        <td>ЕВСЕЕВА Г. Г</td>
                <td>РОС9240263989</td>
</tr>
<tr class='hideit'>
                <td>6  </td>
                <td>4</td>
        <td>ХОЛОДНЯКОВА-2 С. С</td>
                <td>РОС9240263986</td>
</tr>
<tr class='hideit'>
                <td>7  </td>
                <td>5</td>
        <td>БРИЧКИНА Н. Е</td>
                <td>РОС9240263987</td>
</tr>
<tr class='hideit'>
                <td>8  </td>
                <td>5</td>
        <td>ГОЛОВКОВА Н. С</td>
                <td>РОС9240263988</td>
</tr>
<tr class='hideit'>
                <td>9  </td>
                <td>6</td>
                <td>>ГУЩИНА Л. М</td>
</tr>
<tr>
                <td>10  </td>
                <td>3</td>
        <td>ЯСИНСКАЯ-4 И. В</td>
                <td>СТА6240263578</td>
</tr>
<tr class='hideit'>
                <td>11  </td>
                <td>4</td>
        <td>МИЩЕНКО И. Н</td>
                <td>СТА6240263579</td>
</tr>
<tr class='hideit'>
                <td>12  </td>
                <td>4</td>
        <td>ТКАЛЕНКО Т. П       </td>
                <td>СТА6240263670</td>
</tr>
<tr class='hideit'>
                <td>13  </td>
                <td>5</td>
        <td>ДЬЯКОНОВА О. Ю</td>
                <td>СТА1180273916</td>
</tr>
<tr>
                <td>14  </td>
               ...

JavaScript

var table = document.getElementById("mytable");

var trs = table.getElementsByTagName("tr");
var hideit_length = 0;
for(var i=0;i<trs.length;i++){
    if(trs[i].className == "hideit"){ 
        hideit_length++;
    }else{
        if(hideit_length != 0)alert(hideit_length);
        hideit_length = 0;
    }
}