CrossoverIndValue-indicator-number-of-days-it-s-true

You didn't really say what "true" was, but since you wanted to use CrossoverIndValue, I assumed that it mean the number of days since crossing over the high value until the crossing under the low value.

CODE:
using WealthLab.Backtest;
using System;
using WealthLab.Core;
using WealthLab.Data;
using WealthLab.Indicators;
using System.Collections.Generic;

namespace WealthScript3 
{
    public class MyStrategy : UserStrategyBase
    {

        public override void Initialize(BarHistory bars)
        {
         _rsi = RSI.Series(bars.Close, 14);
         PlotIndicator(_rsi);
         
         double xvalue = 55; 
         _ind1 = new CrossoverIndValue(bars, _rsi, xvalue, 30.00);            
         PlotIndicator(_ind1, WLColor.FromArgb(255,107,139,139), PlotStyle.Line);

         _barsSince = TimeSeries.BarsSince(_ind1 < 0);
         PlotTimeSeriesLine(_barsSince, $"Bars Since RSI CrossOver {xvalue}", "BarsSincePane");
         SetPaneMinMax(_ind1.PaneTag, -1.2, 1.2);
        }


        public override void Execute(BarHistory bars, int idx)
        {
        }

      TimeSeries _barsSince;
      IndicatorBase _ind1;
      RSI _rsi;
      
    }
}